Investigation and control of a meticillin-resistant Staphylococcus aureus (MRSA) outbreak in a Level 2 neonatal unit in England: findings from a cohort study - PMC Skip to main content An official website of the United States government Here's how you know Here's how you know Official websites use .gov A .gov website belongs to an official government organization in the United States. Secure .gov websites use HTTPS A lock ( Lock Locked padlock icon ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites. Search Log in Dashboard Publications Account settings Log out Search… Search NCBI Primary site navigation Search Logged in as: Dashboard Publications Account settings Log in Search PMC Full-Text Archive Search in PMC Journal List User Guide PERMALINK Copy As a library, NLM provides access to scientific literature. 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Learn more: PMC Disclaimer | PMC Copyright Notice Infect Prev Pract . 2026 Mar 12;8(2):100532. doi: 10.1016/j.infpip.2026.100532 Search in PMC Search in PubMed View in NLM Catalog Add to search Investigation and control of a meticillin-resistant Staphylococcus aureus (MRSA) outbreak in a Level 2 neonatal unit in England: findings from a cohort study K Thorley K Thorley a Health Protection in Regions Directorate, UK Health Security Agency, London, UK b UK Field Epidemiology Training Programme, UK Health Security Agency, London, UK Find articles by K Thorley a, b, ⁎ , J Foster J Foster c Mid and South Essex NHS Foundation Trust, East of England, Essex, UK Find articles by J Foster c , S Urquhart S Urquhart c Mid and South Essex NHS Foundation Trust, East of England, Essex, UK Find articles by S Urquhart c , J Swinney J Swinney c Mid and South Essex NHS Foundation Trust, East of England, Essex, UK Find articles by J Swinney c , NQ Verlander NQ Verlander d Statistics, Modelling and Economics Department, UK Health Security Agency, London, UK Find articles by NQ Verlander d , E Clissold E Clissold e Royal Free London NHS Foundation Trust, London, UK Find articles by E Clissold e , M Ganner M Ganner f Antimicrobial Resistance and Healthcare Associated Infections Division, UK Health Security Agency, London, UK Find articles by M Ganner f , Y Ryan Y Ryan f Antimicrobial Resistance and Healthcare Associated Infections Division, UK Health Security Agency, London, UK Find articles by Y Ryan f , K Moganeradj K Moganeradj f Antimicrobial Resistance and Healthcare Associated Infections Division, UK Health Security Agency, London, UK Find articles by K Moganeradj f , T Lamagni T Lamagni f Antimicrobial Resistance and Healthcare Associated Infections Division, UK Health Security Agency, London, UK Find articles by T Lamagni f , E Dowling E Dowling c Mid and South Essex NHS Foundation Trust, East of England, Essex, UK Find articles by E Dowling c , S Kapadia S Kapadia a Health Protection in Regions Directorate, UK Health Security Agency, London, UK Find articles by S Kapadia a , E Heinsbroek E Heinsbroek a Health Protection in Regions Directorate, UK Health Security Agency, London, UK Find articles by E Heinsbroek a Author information Article notes Copyright and License information a Health Protection in Regions Directorate, UK Health Security Agency, London, UK b UK Field Epidemiology Training Programme, UK Health Security Agency, London, UK c Mid and South Essex NHS Foundation Trust, East of England, Essex, UK d Statistics, Modelling and Economics Department, UK Health Security Agency, London, UK e Royal Free London NHS Foundation Trust, London, UK f Antimicrobial Resistance and Healthcare Associated Infections Division, UK Health Security Agency, London, UK ⁎ Corresponding author. Address: Health Protection in Regions Directorate, UK Health Security Agency, 61 Colindale Avenue, London NW9 5EQ, London, UK. [email protected] Received 2025 Nov 25; Accepted 2026 Mar 3; Collection date 2026 Jun. Crown Copyright © 2026 Published by Elsevier Ltd on behalf of The Healthcare Infection Society. PMC Copyright notice PMCID: PMC13098614 PMID: 42022787 SUMMARY Background Neonatal unit outbreaks of meticillin-resistant Staphylococcus aureus (MRSA) can be challenging to control; understanding risk factors for transmission is important. This paper presents findings from a neonatal unit MRSA colonization outbreak investigation, detected through routine screening. Aim To describe the outbreak investigations, and risk factors identified through a cohort study. Methods Cases were neonatal unit inpatients from May to September 2023 with an MRSA-positive sample within the whole-genome sequencing (WGS)-linked outbreak cluster. Concurrent screening of mothers was conducted throughout, along with one-off staff and environmental sampling. Hospital record data for all unit inpatients between May and September were reviewed. A retrospective cohort study was conducted. Findings Of 84 neonatal unit inpatients, 14 (16.7%) had MRSA colonizations within the WGS-linked cluster. There were two outbreak waves: seven cases in outbreak weeks 1–8, and seven cases in outbreak weeks 16–19. The median time between admission and first MRSA-positive swab was 9.5 days (range 3–24). In univariable analysis, MRSA colonization was associated with lower gestational age, lower birth weight, and having clinical exposures (respiratory support, invasive lines, and gavage or enteral feeding). No association was identified with twin birth, delivery method or cot exposures. Maternal and environmental screening identified no MRSA-positive results. Staff screening identified two outbreak-strain-positive results. Conclusions This paper describes an outbreak of MRSA colonizations in a neonatal unit in England, and the risk factors for infant colonization, identified by conducting a cohort study. The outbreak, whilst complex, was controlled successfully, with the second outbreak wave guiding implementation of additional control measures, including staff screening. Keywords: Healthcare-associated infections, Outbreak, Neonates, MRSA Introduction Neonatal care units are challenging for outbreak control due to the clinically vulnerable population and complex care required. Meticillin-resistant Staphylococcus aureus (MRSA) is a frequent cause of neonatal unit outbreaks, causing severe infections, including sepsis [ 1 , 2 ]. MRSA colonization (i.e. presence of the organism without signs of infection) is associated with increased risk of subsequent infection [ 3 ]. Therefore, identifying and controlling MRSA colonization outbreaks promptly is important to prevent MRSA infection among neonates in care units. In England, routine weekly MRSA screening is recommended by regional neonatal networks [ 4 ]. This can be used as a tool for early outbreak detection, and supplemented by whole-genome sequencing (WGS) to characterize potential outbreak strains and transmission pathways [ 5 ]. Wider screening, including staff and parents, in conjunction with WGS supports understanding of the full extent of the outbreak, and provides further granularity of possible transmission routes [ 5 , 6 ]. Environmental MRSA reservoirs are also of concern during outbreaks, with investigations often isolating the outbreak strain from environmental sampling [ 7 , 8 ]. Other investigations have described uneven distributions of cases across neonatal unit rooms and cots, suggesting transmission linked to local environmental contamination [ 9 ]. Identification of risk factors in outbreak settings is important to guide control measures. Several risk factors for MRSA colonization in neonatal units have been identified, including low birth weight, multiple gestation, and use of antibiotics [ 10 , 11 ]. Evidence is mixed for other possible risk factors, including gestational age, delivery method, and length of stay [ 10 ]. However, limited studies have investigated risk factors in outbreak settings using analytical epidemiological methods [ 10 ]. This paper describes a neonatal care unit outbreak of MRSA colonization in England, and the risk factors associated with colonization identified using a cohort study. Methods Setting The outbreak occurred on a Level 2 neonatal unit in a district general hospital in England [ 12 ]. Whilst Level 2 units in England typically provide care for babies born at ≥27 weeks of gestation, at the time of the outbreak, the unit accepted admissions from infants born at ≥32 weeks of gestation; infants born at younger gestational ages could be repatriated after receiving initial care at neighbouring units. The neonatal unit had a 16-cot (including incubators and bassinet cots) capacity, delivering intensive, high-dependency and special care across three rooms. There were three single-occupancy side rooms and one parent room in use. Nursing staff would typically provide care within a single room each day, whilst doctors and other allied healthcare professionals would provide care or support across rooms. MRSA screening Nose, ear and perineal swabs for MRSA were taken from infants on admission, weekly thereafter, and on discharge. For infants repatriated from other hospitals, five separate admission swabs were taken (nose, throat, axilla, groin, ear). If an infant tested positive, the mother was also screened (nose, groin). The hospital laboratory cultured isolates, without a broth enrichment step, using Oxoid Brilliance ™ MRSA 2 agar, conducted antimicrobial susceptibility testing using EUCAST breakpoints [ 13 ], and sent isolates from suspected outbreaks to the Staphylococcus Reference Service (SSRS) for WGS. Outbreak detection An MRSA colonization was identified during weekly screening; when two further cases of colonization were identified in different infants during the next weekly screening round, an outbreak investigation was initiated. The hospital infection prevention and control (IPC) team implemented immediate control measures. WGS of the culture-positive specimens confirmed that isolates were closely genetically related. An incident management team (IMT) was established to guide the outbreak response; attendants included the neonatal unit staff, hospital and trust-level IPC team and nursing and neonatal directors, clinical microbiologists, occupational health, and the UK Health Security Agency (UKHSA) regional health protection team and field service. Outbreak response Control measures Control measures were determined by the IMT and implemented by the unit staff and hospital IPC team. Support from UKHSA was requested in June 2023, with additional epidemiological support requested in September 2023. Microbiological investigations All culture-positive specimens were referred to the SSRS for WGS. WGS was performed on purified bacterial DNA using Illumina's short read sequencing technology on NextSeq 1000, and processed as described by Utsi et al. [ 14 ]. Species-level contamination checks were conducted using KmerID (available: https://github.com/ukhsa-collaboration/kmerid ), and a minimum read depth of 30 and yield of 150 mb were required. Sequences were deposited in a public repository (PRJEB105409). Genetic relatedness was determined using multi-locus sequence typing (MLST) [ 15 ] and single nucleotide polymorphism (SNP) distance thresholds, with a cut-off of 5 SNPs. SnapperDB version 3 [ 16 ] was used to perform SNP distance determination [ 16 ] (Supplementary A6). Areas of recombination and mobile elements present in the reference were removed prior to application of the SNP threshold. A phylogenetic tree was created using itol, from gubbins (version 3.4) final_tree.tree [ 17 ], with default settings. Unit staff were screened once (nose, groin), supported by occupational health. Environmental screening was conducted once by the IPC team, with the swabs cultured on MRSA-specific media (Oxoid Brilliance ™ MRSA 2 agar). Epidemiological investigations Cases were defined as infants admitted to the unit with MRSA-positive samples within the genetically-linked outbreak cluster from May 2023. A line list was maintained including patient identifiers, demographics, admission information and microbiological results from the UKHSA laboratory record system and reference laboratory. A descriptive summary of all cases was produced, including an epidemiological curve. Time between admission and the first MRSA-positive swab date was described and summarized graphically. Case detections over time were compared with staff movements (days worked on the unit) for any staff members with MRSA colonization detected during staff screening days. This cohort study was conducted to identify risk factors for MRSA colonization in the outbreak. The cohort was defined as infants with an admission to the unit during the time between the specimen dates of the first and last cases, and who were admitted during at least one routine weekly screening event (Mondays, 6 am–12 pm). The data were identified from electronic patient records provided by the hospital informatics team. Negative screen data were unavailable. The outbreak line list was used to identify outbreak cases among the cohort. Infants with positive screens of non-outbreak MRSA strains were excluded from the cohort. Directed acyclic graphs (DAGs) were produced using DAGitty ( www.dagitty.net ) to guide data collection, informed by expert knowledge and a rapid literature review ( Supplementary Figure A1 ). Information was collected on sex, birth weight, gestational age, delivery methods, clinical procedures, and admission dates (all available electronically), and twin birth and cot movements (available in patients' paper notes). Patients' notes were reviewed manually for all cases and a subset of non-cases; among the non-cases, 42 infants (three per case) were selected for review using simple random sampling without replacement. Gestational age and birth weight were classified according to the World Health Organization standard thresholds [ 18 ]. Clinical procedure codes were grouped into clinical risk factors: receiving any type of respiratory support (including ventilation, continuous positive airway pressure, and other respiratory support codes); insertion of invasive lines; gavage or enteral feeding; and invasive diagnostic procedures. Clinical procedure dates were unavailable. Cot exposures were defined as occupying a cot which an MRSA-colonized infant had occupied during the previous 14 days, during the admission (non-cases) or prior to a positive MRSA swab (cases). Time at risk was defined as the total length of admission for non-cases, and length of admission prior to the positive MRSA swab date for cases. As individual staff/infant interactions were not collected, individual staff exposures were not included in the model. Firth's logistic regression for univariable and multivariable analysis was used to account for small case numbers. Multi-variable regression was conducted using a forward stepwise approach; variables with a P -value <0.2 for any category in univariable analysis were considered and added in order of increasing significance level. Collinearity was assessed using Cramer's V, and one variable was dropped where strong, following discussion with the study team. Models were compared using penalized likelihood ratio tests, with the final model selected when addition of further variables showed no evidence of model improvement or did not substantially change (>20%) the odds ratios (OR). A sensitivity analysis was conducted for univariable analysis, first restricting the minimum length of admission for non-cases to the minimum time to MRSA detection for cases (3 days), then including only those non-cases where manual notes had been reviewed, and finally removing the cohort criteria requiring all infants to be on the unit during a weekly screening event. All analyses were conducted in RStudio version 4.3.1. Results Description of the outbreak and characteristics of the outbreak strain Fourteen infant outbreak cases were identified. Two cases had eye infections in addition to MRSA colonization; no other infants had clinical signs of infection, and there were no invasive infections. The outbreak MRSA isolates represented a single staphylococcal lineage with a novel MLST (closest to ST:8), and formed a single 5-SNP cluster, suggestive of linked transmission events, which was supported by visualization of the phylogenetic relationship between isolates ( Figure 1 ). One unrelated MRSA colonization was detected. In addition to meticillin resistance, where sensitivities were available, outbreak isolates were resistant to ciprofloxacin (13/13), gentamicin (14/14) and trimethoprim (13/13); isolates were sensitive to clindamycin (14/14), erythromycin (12/12), mupirocin (14/14) and tetracycline (12/12). Figure 1. Open in a new tab Phylogenetic tree of infant and staff outbreak meticillin-resistant Staphylococcus aureus isolates. Where more than one isolate was sequenced for an individual, isolates are numbered following the ID. Following the three cases that prompted the outbreak investigation, four more cases of colonization were identified up to week 8 of the outbreak, all with stays overlapping with at least one other case ( Figures 2 and 3 ). Between weeks 9 and 15, there were no further cases despite continuing admissions to the unit. A second outbreak wave occurred from week 16, with seven further colonizations identified up to week 19, all with stays overlapping with at least one other case ( Figures 2 and 3 ). During the 7-week period with no new cases detected, only one MRSA-colonized infant was on the unit, indicating minimal overlap of infants on the unit between the first and second outbreak waves ( Figure 3 ). Figure 2. Open in a new tab Epidemic curve of meticillin-resistant Staphylococcus aureus colonization outbreak cases among infants, by outbreak week of first positive sample. Figure 3. Open in a new tab Timeline of infection clusters plot showing time period of hospitalization in the neonatal unit for meticillin-resistant Staphylococcus aureus (MRSA) colonization outbreak cases among infants. Bars represent admission period, circles represent the date of the first positive MRSA specimen. Ordered and numbered by date of admission to unit. The observed median time to MRSA colonization detection from admission was 9.5 days (range 3–24 days). Seven cases (50%) were female and almost all (13/14) were born before 37 weeks of gestation. Eleven cases (79%) had low birth weights (<2500 g). Most cases were born in the associated hospital maternity unit (11/14, 79%) and delivered via caesarean section (10/14, 71%). Three cases (21%) had second admission periods, after short first admissions, reflecting transfers after birth to other units for their initial care period ( Figure 3 ; infants #7, #8 and #11). Cases were admitted to the outbreak neonatal unit for a median of 2.4 weeks (interquartile range 1.8–4.7 weeks). During the second outbreak wave, all staff members on the unit were invited to participate in staff screening. Ninety-two staff were identified by occupational health, with swabs (nose, groin) received from 88 staff members; samples from two were positive and WGS-linked to the outbreak cluster. Both staff members (referred to as A and B) completed decolonization treatment successfully and returned to work. Staff members A and B worked on the unit across the outbreak period, including the period with no case detections. Nose and groin swabs were taken from the mothers of 12 of the 14 MRSA-colonized infants; all were negative for MRSA. During the second wave, 58 environmental swabs of key unit locations ( Supplementary Table A2 ) were taken on a single occasion; all were negative for MRSA. Cohort study Overall, 128 infants were admitted during the outbreak period, of which 84 were included in the cohort study; 14 cases and 70 non-cases (overall attack rate 16.7%). Forty-three non-cases were excluded due to not being on the unit during a weekly screening event. One further infant with a non-outbreak strain MRSA colonization was excluded from the cohort. Manual data collection, required for cot movements and twin birth, was completed successfully for all cases and 31 non-cases. MRSA colonization was associated with younger gestational age, having clinical exposures (respiratory support, invasive lines, and gavage or enteral feeding), and low birth weight ( Table I ). There was some evidence in univariable analysis that infants with a time at risk of 1–2 weeks had higher odds of MRSA colonization compared with those admitted for <1 week ( P =0.068). There was no evidence of an association with sex, twin birth or delivery method ( Table I ). Additionally, no association was found with occupying a cot (bassinets and incubators) where an MRSA-colonized infant had been within the previous 14 days. Table I. Univariable and multi-variable (adjusted) association between case (infants with meticillin-resistant Staphylococcus aureus colonization)/non-case status and demographic, clinical and other exposures Open in a new tab None of the sensitivity analyses detected substantial differences in associations ( Supplementary Tables A3–A5 ). In the sensitivity analysis removing the criteria requiring all infants to be on the unit during a weekly screening event, a stronger association was observed for infants with a time at risk of 1–2 weeks compared with those admitted for <1 week ( Supplementary Table A5 ). The final multi-variable model included gestational age and respiratory support. Gestational age and birth weight were strongly collineated variables (Cramers V: 0.718; Chi-squared P -value <0.001), and therefore gestational age alone was included in the model, decided a priori . Receiving respiratory support was a risk factor independent of gestational age; infants receiving respiratory support had 4.1 times higher odds of having MRSA colonization ( Table I ). Gestational age was associated with increased odds of MRSA colonization, having adjusted for respiratory support; infants born at <32 weeks of gestation had 9.6 times higher odds of having MRSA colonization during this outbreak compared with full-term infants (≥37 weeks), whilst infants born between 32 and <37 weeks of gestation had 5.3 times higher odds ( Table I ). Implementation of control measures During the first outbreak wave, an IMT was convened. A review of cases' common exposures was undertaken, including staffing rotas, incubator movements, and other unit visitors who utilize equipment for multiple infants during a session (e.g. for ophthalmology assessments). Decolonization regimens for MRSA-positive infants were discussed, and a standard approach was recommended using mupirocin nasal ointment for 5 days and antiseptic body wash containing octenidine for 5 days, followed by 2 days of normal wash, with swabbing repeated until discharge. Staff screening was discussed; however, other control measures were implemented first, in line with recommendations [ 19 ]. Cleaning procedures were reviewed, including the introduction of double cleaning for incubators. Terminal cleans were conducted following discharge or movement of infants with known MRSA colonization. Single-use items, or items allocated to single infants, were used during the outbreak to reduce possible contamination by crossover. The unit was reviewed for opportunities to reduce clutter and increase cleaning efficiency, with additional storage facilities identified outside the unit. Maintenance work was also conducted on unit water pipes to increase flow to clinical handwash basins. The IPC team conducted observational assessments and hand hygiene audits, finding high compliance. Both handwashing basins and alcohol-based hand rub were in use. Skin conditions among staff were considered, with no concerns reported to the IMT. In response to the outbreak, all staff were required to wear scrubs on the unit. Handwashing training was provided to staff and visitors, and measures were taken to reduce footfall through the unit. During the second outbreak wave, the situation was reviewed to identify additional control measures, including one-off staff and environmental screening. Both staff members with MRSA colonization detected during screening completed decolonization regimens successfully and returned to work. The unit was closed temporarily to facilitate further maintenance work, including replacement of all handwashing sinks, addition of a new sink at the unit entrance, and additional storage facilities for parents. One-off fogging with hydrogen peroxide vapour and ultraviolet cleaning were conducted while the unit was unoccupied. Following the terminal clean and renovations, a final IMT meeting was held in October, with the unit reopened to admissions 4 weeks later. Weekly MRSA screening was continued on the unit, as per routine protocol. Discussion This paper describes an outbreak of MRSA colonization in a neonatal unit in England, and the risk factors for infant colonization, identified by conducting a cohort study. During the outbreak, characterized by two outbreak waves spanning 19 weeks, 14 infants with MRSA colonization were detected. Analytical epidemiological studies are not conducted frequently in hospital-based outbreak settings; this investigation supports outbreak-specific risk factor characterization and contributes to the wider evidence base for outbreak response within this clinically vulnerable population. The two outbreak waves suggested a persistent environmental or staff MRSA reservoir during the period with no cases, which guided control measures implemented during the second outbreak wave. The persistence and re-emergence of outbreak strains following a period of no cases has been observed during other MRSA outbreaks, indicating the challenges of identifying persistent infection reservoirs [ 9 , 20 , 21 ]; Harris et al. reported a 64-day gap between MRSA cases in a special care unit outbreak [ 5 ]. Routine weekly infant screening for MRSA was important in identifying this outbreak and the re-emergence of cases. Outbreak-linked staff colonization has been identified previously [ [20] , [21] , [22] , [23] ]. Staff screening can be logistically intensive and, if not conducted sensitively, can potentiate guilt and feelings of blame among staff members. The decision to conduct staff screening in this outbreak followed the epidemiological evidence of a persistent environmental reservoir, despite having implemented immediate and varied control measures during the first outbreak wave [ 19 ]. As granular data on staff/infant interactions were not collected, the authors were not able to include staff exposures in the cohort analysis, and therefore were not able to provide conclusions on, or infer, the direction of transmission between staff and infants. However, the identification of staff colonization with the outbreak strain highlights the potential contribution of a staff reservoir to the re-emergence of the outbreak. Staff screening and decolonization were implemented successfully, as recommended by the IMT, supporting its use in this setting. In previous outbreaks, staff attitudes to screening or treatment approaches have been assessed; attitudes and experiences of staff screening could have been explored here, and would benefit the future implementation of such interventions [ 24 ]. Additionally, a review of staff screening conducted in different neonatal unit outbreak responses would be beneficial to inform recommendations in outbreak response situations, including on the timing of screening. In this study, gestational age, low birth weight, and clinical exposures were associated with MRSA colonization in univariable analysis. No association was found with delivery method and sex, in agreement with previous studies and evidence reviews [ 10 , 11 , 22 ]. However, in contrast to previous studies, the present study did not find an association with twin birth [ 10 , 22 ]. Both gestational age and birth weight have been associated with risk of MRSA acquisition previously in a similar setting [ 22 ]; due to the number of cases restricting the power of the present study, the authors were unable to disentangle the associations. These findings support that factors, such as gestational age, can be used as a proxy indicator for clinical teams to identify the most vulnerable infants during an outbreak, although it is acknowledged that the association is likely to reflect differences in later exposures, supported by the use of DAGs in the analysis. A limitation of this cohort analysis was not having negative screening data available; therefore, being on the unit during a weekly screening event was applied as a proxy for non-cases. This methodology was required to identify infants with the opportunity to become cases; infants with shorter stays were more likely to be excluded using this method. Among clinical exposures, receiving respiratory support was most strongly associated with MRSA colonization, with the association independent of gestational age. In a previous systematic review, clinical exposures such as underlying medical conditions, invasive procedures, and indwelling medical devices such as catheters were not strongly associated with MRSA colonization [ 11 ]. Respiratory support requires frequent and high levels of contact with medical/nursing professionals, and therefore the increased risk observed in this study may reflect increased staff contact. This finding further supported implementation of staff screening in this setting. Environmental MRSA exposures were investigated. Previous investigations have identified outbreak strains during environmental sampling [ 7 , 8 ]. Staphylococcus capitis has been isolated previously from the incubators of cases and nearby bed bays, and incubator disinfection failures were identified [ 25 ]. Therefore, whilst no environmental samples detected MRSA, this study explored whether occupying a cot previously occupied by an MRSA-colonized infant was a risk factor; however, no association was found. Double incubator cleaning was implemented during the outbreak; not detecting an association with cot exposures suggests that this may have been a successful control measure. The authors were unable to explore the proximity of infants on the unit, due to data constraints; uneven distribution of cases in neonatal unit rooms has been observed previously [ 9 ]. Microbiological methods vary, and in this setting, no broth enrichment step for MRSA swab processing is used, which could impact the detection sensitivity for environmental and other screening. The observed time between admission and MRSA detection in outbreak settings has not been reported frequently; the time to detection in this outbreak, where weekly screening was conducted (median 9.5 days, range 3–24 days), appeared shorter than in a previous non-outbreak setting, where Nurjadi et al. reported median time to first detection as 17 days (interquartile range 11–37) [ 26 ]. In the present outbreak setting, some evidence was found supporting an association between time at risk and likelihood of colonization. In another outbreak setting [ 8 ], days of exposure to MRSA was found to be a risk factor for MRSA acquisition in the unadjusted analyses but not the adjusted analyses, but this has been observed inconsistently, including in non-outbreak settings, and with varying methodologies [ 27 , 28 ]. Whilst time at risk was considered in this study, several factors have been shown to predict overall length of admission, including gestational age [ 29 ]. As this outbreak involved colonization, the time to detection will, in part, reflect the timing of screening rather than exact acquisition time. This cohort study was initiated due to the prolonged length of the outbreak, and supported the identification of outbreak-specific risk factors. However, data collection to facilitate analytical studies can be challenging and time-consuming, making delivery of study findings within the outbreak period challenging; capacity to support these investigations can be limited. Strong collaboration between the multiple specialist teams involved in this outbreak facilitated completion of this study. The authors aim to share the findings across other similar neonatal unit settings who may experience outbreaks with common characteristics. In conclusion, this neonatal unit MRSA colonization outbreak, whilst complex, was controlled successfully; no cases of bacteraemia occurred, and no outbreak strain cases were identified after control measures were implemented by collaborative working between specialist teams. The suite of control measures which controlled this outbreak reflect the complexity of MRSA outbreak control. The cohort study characterized risk factors in the outbreak setting. Identification of staff MRSA colonization and the association with respiratory support, requiring high levels of contact care, provide support for staff screening as an intervention to support outbreak control in persisting situations. CRediT authorship contribution statement K. Thorley: Writing – original draft, Visualization, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. J. Foster: Writing – review & editing, Resources, Investigation, Data curation. S. Urquhart: Writing – review & editing, Resources, Investigation, Data curation. J. Swinney: Writing – review & editing, Resources, Investigation, Data curation. N.Q. Verlander: Writing – review & editing, Methodology. E. Clissold: Writing – review & editing, Investigation. M. Ganner: Writing – review & editing, Investigation. Y. Ryan: Writing – review & editing, Investigation. K. Moganeradj: Writing – review & editing, Investigation. T. Lamagni: Writing – review & editing, Methodology. E. Dowling: Writing – review & editing, Investigation, Conceptualization. S. Kapadia: Writing – review & editing, Supervision, Methodology, Investigation, Conceptualization. E. Heinsbroek: Writing – review & editing, Supervision, Methodology, Investigation, Conceptualization. Ethics UKHSA has legal permission to process patient confidential information for public health purposes under Section 2A of the National Health Service Act 2006 (Secretary of State's duty as to protection of public health) and Regulation 3 of the Health Service (Control of Patient Information) Regulations 2002 (Communicable diseases and other risks to public health). Funding sources None. Conflict of interest None declared. Acknowledgements The authors wish to recognize the contribution of the IMT, the neonatal unit staff, the NHS trust informatics team, the hospital occupational health team and the hospital laboratory department. Additionally, the authors wish to recognize the contributions of Michelle Toleman, UKHSA Field Services, and support and supervision of Mari Morgan under the UK Field Epidemiology Training Programme. Footnotes Appendix A Supplementary data to this article can be found online at https://doi.org/10.1016/j.infpip.2026.100532 . Appendix A. 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