How do general approaches to an epidemic impact infection control?

Healthcare associated infections (HAIs) are an important source of morbidity and mortality in hospitals. An important minority of these infections occur as part of an outbreak or cluster (occurrence of infections at a rate statistically significantly higher than background rates of infection). When a cluster is suspected, it is crucial that the investigation occur in a systematic fashion. This will confirm whether an outbreak is really occurring (versus a pseudo outbreak), assist with hypothesizing potential causes of the outbreak and suggest control measures. Several factors may be used to decide how extensive an investigation is needed. In many cases, the outbreak will be eradicated with simple control measures, in other cases, more extensive epidemiological investigation (i.e., comparative studies, observational studies, environmental studies, and microbiological/molecular typing studies) is perhaps needed. A series of factors, including resources may guide the decision to investigate an outbreak more extensively.

Although a minority of the estimated greater than 2 million HAIs in the United States annually likely occur as clusters/outbreaks, investigation of some of these situations can adds to the collective knowledge of the infection control and healthcare epidemiology community. These potentially include informing about new pathogens, new presentations of older pathogens, new at risk populations, demonstrate effective investigation, laboratory, statistical or control strategies identify regional or national outbreaks, new vehicles or mechanisms of transmission.

What elements of these approaches are necessary for infection prevention and control?

An organized step-by-step approach is essential in the investigating of healthcare associated outbreaks. Although steps are presented in a specific order, many steps may occur simultaneously and some steps may not be needed.

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A stepwise approach to outbreak investigations includes:

  • Surveillance

  • Detection of a “problem”

  • Determining existence of an outbreak

  • Excluding pseudo outbreak

  • Defining case

  • Case ascertainment

  • Comparing pre-epidemic and epidemic period rates (confirming existence of an outbreak)

  • Epidemiological studies

  • Line listing (a listing of cases and limited factors to assist in generating hypothesis of etiology)

  • Epidemic curve

  • Hypothesizing for a cause

  • Comparative studies (risk factor assessment)- optional

  • Case Control

  • Cohort study

  • Additional studies-optional

  • Review practices

  • Review of literature for plausible etiologies/mechanisms

  • Observational studies

  • Culture surveys

  • Isolate typing

  • Interventions/control measures

  • Assess interventions

  • Sharing findings

What are the consequences of ignoring these approaches?

Failure to use this stepwise approach to outbreak investigation may:

  • Delay identifying an outbreak (allowing ongoing patient morbidity or mortality)

  • Waste resources (i.e., skipping steps leading to pursuing pseudo outbreaks or unlikely sources)

  • Delay in identifying the etiology for infections

What information supports the research regarding general approaches to an epidemic?

Experts from the Centers for Disease Control and Prevention (CDC), healthcare epidemiology and infection control community and local health department’s personnel use this stepwise approach to assess clusters and outbreaks on a regular basis. Many outbreaks may be self limited and resolve on their own, others may respond to a series of control measures. In many cause the exact etiology may not be known. For most problems a more extensive investigation (including advanced epidemiological studies) may not be warranted; however when it is, having a systematic approach is important.

Summary of current controversies.

One area of controversy is which clusters/outbreaks require a more extensive approach. If additional epidemiological studies are going to be performed, which is the best comparative study? The two most commonly used studies are case-control studies and cohort studies (contrasted below). If a case control study is chosen, controversies include which is the best control group for comparison and which variables and for how long variable exposure should be collected. Once a study is conducted, an area of controversy is how best to adjust for confounding factors, such as severity of illness and length of stay (that includes matching and stratified analysis). In terms of control measures, controversy also surrounds the criteria for closing and (re-opening a unit). Finally there are ethical concerns in outbreak investigations.

What is the impact of these approaches relative to the impact of other aspects of infection control?

Most healthcare associated infections occur sporadically and not as part of an outbreak. Outbreaks, however, can be severe, garner attention and cause fear.

Investigating outbreaks may add to the knowledge about (including, but not limited to):

  • The pathogenesis of emerging pathogens (new pathogens, older pathogen causing different symptom presentation, disease in a different population that previously identified to be at risk)

  • Plausible mechanisms of infections

  • New statistical or novel epidemiological studies

  • New laboratory testing/techniques

  • Illustrate newer infection control and other control measures

  • Identify regional or national outbreaks (beyond one facility, such as intrinsic contamination of medications)

  • The need for newer surveillance systems

  • The need for newer guidelines and regulations

Overview of important clinical trials, meta-analyses, case control studies, case series, and individual case reports related to infection control and general approaches to an epidemic.

The stepwise approach to outbreak investigations has been used by public health and healthcare epidemiology/infection control experts. Table I contains the resources, summaries and outbreak reports that illustrate this strategy for outbreak investigation and control of healthcare associated infections.

Controversies in detail.

Controversy 1–After the initial steps of an outbreak are performed, one may need additional studies to identify the source of an outbreak. Settings in which these additional efforts should be considered are:

  • Resources are available

  • The outbreak is associated with severe disease or high mortality

  • New or unusual pathogens or mode of transmission are identified

  • Outbreak continues despite control measures

Controversy 2–The choice of best comparative study depends on resources, time and size of outbreak. The two most commonly used studies are case-control studies and cohort studies (contrasted below):

  • Case Control Study: Case patients are compared to control patients who do not have the adverse outcome or infection, but would have had the opportunity for exposure.

  • Advantages: can be used when outbreaks are large, when long periods of exposures are present, and when many potential exposures need to be reviewed.

  • Disadvantages: only establishes that cases were more likely than controls to have been exposed to potential risk factors (calculates an odds ratio).

  • Controversies: best control group (including whether some cases and controls should be matched on some factors), how long variables of interest should be collected in case and control patients, choosing which variables should be examined (only those clinically and biologically plausible, due to the possibility of falsely- positive results/associations, known as a “type I” error).

  • Cohort Study: Assesses in a forward direction of time the entire population and evaluates what exposures are more common among those who develop disease or infection versus those who do not.

  • Advantages: Unlike a case-control study, a relative risk can be calculated which shows an exposure is not just associated, but denotes causation

  • Disadvantages: More time consuming, costly, fewer factors can be examined

Controversy 3– Once a study is conducted and analysis is planned, one area of controversy is how best to account for the influence of confounding factors such as severity of illness and length of stay. One method is matching the controls to the case on these factors. The disadvantage to this method is that once there is matching for a factor, that factor can no longer be evaluated. Another method is to statistical adjusted in the analysis after data collection by doing a stratified analysis or multivariate analysis.

Controversy 4– when to perform environmental cultures. For the most part, environmental cultures should be guided by the facts of the investigation, data from analyses and biological plausibility. Non directed cultures can waste resources and could yield positive results that would lead the investigation in a wrong direction. In a similar fashion, healthcare worker cultures should be not be routinely performed but should be guided by epidemiology. A culture could be positive, but not have any relationship to cases [i.e., a healthcare work in a neonatal intensive care unit (NICU) may have cultures positive for methacillin resistant S. aureus (MRSA), but that doesn’t necessarily mean that this is the cause of MRSA in the NICU babies]. Thus positive cultures could adversely affect the healthcare worker including unnecessary reassignment or removal from clinical duties or stigma.

Controversy 5–When clusters appear to affect patients in a specific unit or population (i.e., patients undergoing specific surgery, neonates) there may be pressure, including from administrators to close that unit or stop that type of surgery. It is important that these decisions are driven by data, can be justified, and that specific criteria are set for when to close, but more importantly what would need to happen to allow the area/procedure to open/resume.

Controversy 6–Ethical issues potentially arise during outbreak investigation. Is it worth the resources (i.e., time, staff that is diverted from other activities, monetary costs) to do a more extensive investigation and when is the threshold for doing so? The above discussion relating to healthcare worker cultures brings up the ethical issue regarding the cultures (i.e., whether workers can refuse, whether it is appropriate to reassign or remove healthcare workers who are implicated by outbreak findings). Another ethical issue is whether patients and families should be informed of the presence of an outbreak and its etiology when discovered.

What other consensus group statements exist, and what do key leaders advise?

The Centers for Disease Control and Prevention has training programs that underscore the specific approach to outbreaks in various healthcare locations. The Society for Healthcare Epidemiology of American (SHEA) and the Association of Professionals in Infection Control and Epidemiology (APIC) have specific texts that underscore this approach, as well as have published specific examples of the use of these steps to identify outbreaks, specific etiologies and control measures.


Beck-Sague, C, Javis, W, Martone, W. “Outbreak investigations”. Infection Control Hosp Epidemiol. vol. 18. 1997. pp. 138-145.

Ostrowsky, B, Jarvis, W. “Challenges in outbreak investigations in intensive care units”. Critical Care infectious diseases textbook. 2001. pp. 377-403.

Ostrowsky, B, Jarvis, W. “Efficient management of Outbreak investigations”. Wenzel's Prevention and Control of Nosocomial Infections. 2003. pp. 500-523.

“Centers for Diseases Prevention and Control (CDC), Outbreak investigation : A Cheat Sheet”. 2011.

Jarvis, W. “Usefulness of molecular epidemiology for outbreak investigations”. Infect Control Hosp Epidemiol. vol. 15. 1994. pp. 500-503.

Archibald, L, Jarvis, W. “The role of the laboratory in outbreak investigations”. Semin Infect Control. vol. 1. 2001. pp. 102-110.

Bredenberg, H, Manangan, L, Jarvis, W. “Selected hospital infection program infections program outbreak investigations, 1994-1998”. Infect Control today. 1999, August. pp. 26-30.

Jarvis, W. “Hospital Infection Program, Centers for Diseases Control and Prevention (CDC) on-site outbreak investigations, 1990 to 1999”. Semin Infect Control. vol. 1. 2001. pp. 73-84.

Jarvis, W. “Noscomial Outbreaks: The Centers for Diseases Control's Hospital Infection Program Experience, 1980-1990”. Am J. Med. vol. 91. 1991. pp. 101S-106S.

Wenzel, R, Thompson, R, Laudry. “Hospital acquired infections in intesnive care unit patients: an overview with emphasis on epidemics”. Infect Control. vol. 4. 1983. pp. 371-375.

Richards, C, Jarvis, W. “Infectious diseases outbreaks in long term care facilities”. Semin Infect Control. vol. 1. 2001. pp. 139-145.

Grohskopf, L, Jarvis, W. “Outbreaks associated with medical devices and medications”. Semin Infect Control. vol. 1. 2001. pp. 111-123.

“Centers for Diseases Prevention and Control (CDC), national Healthcare safety network (NHSN)”.

Weinsten, R. “Stamm W. Pseudo outbreaks in hospital”. Lancet. vol. 2. 1977. pp. 862-864.

Richards, C, Jarvis, W. “Lessons from recent nosocomial epidemics”. Curr Opin Infect Dis. vol. 12. 199. pp. 327-334.

Quesenberry, C. “Statistical process control geometric q- chart for nosocomial infection surveillance”. Am Infect Control. vol. 28. 2000. pp. 314-320.

“Centers for Diseases Prevention and Control (CDC), Update: investigation of anthrax associated with intentional exposure and interim public health guidelines, October 2001”. MMWR. vol. 50. 2001. pp. 1008-1010.

Ostrowsky, B, Whitener, C, Bredenbeg. “Serratia marcescens bacteremia traced to an infused narcotic”. N. Engl. J. Med. vol. 346. 2002. pp. 1529-1537.

Grohskopf, L, Roth, V, Feikin, D. “Serratia liquefaciens-bloodstream infections from contamination of epoetin alfa at a hemodialysis center”. N. Engl. J. Med. vol. 344. 2001. pp. 1491-1497.

“Centers for Diseases Prevention and Control (CDC), Bronchoscopy-related infections and pseudoinfections- New York, 1996 and1998”. MMWR. vol. 48. pp. 557-560.

McDonald, LC, Killgore, G, Thompson, A. “An Epidemic, Toxin Gene–Variant Strain of Clostridium difficile”. N. Engl. J. Med. vol. 353. 2005. pp. 2433-2441.

“Centers for Diseases Prevention and Control (CDC), Transmission of Hepatitis B and C Viruses in outpatient settings– New York, Oklahoma and Nebraska, 2000-2002”. MMWR. vol. 52. pp. 901-906.

“Centers for Diseases Prevention and Control (CDC), Guidelines to Infection Prevention for outpatient settings: Minimum expectations for safe care”.