A systematic review to guide measles exposure periods for contact tracing

Summary

Background

This systematic review examined the duration of measles virus airborne transmissibility after a source case is no longer present to identify evidence gaps in measles contact tracing exposure window guidelines.

Methods

A systematic literature review following PRISMA guidelines was conducted using PubMed, EMBASE, Web of Science, and SCOPUS databases for publications from January 1988 to July 2024. Additional sources were identified through reference list reviews and Google Scholar searches. Studies examining how long the measles virus survives in the air or remains transmissible after an infectious case leaves a public space were included, while editorials, opinion pieces, non-evidence-based recommendations, mathematical models, and publications unrelated to airborne transmission of measles in public environments or not available in English were excluded. Researchers extracted summary data.

Findings

Initial database searches identified 1054 studies, with none meeting initial inclusion criteria after screening. Supplemental searches identified five relevant articles (1964–1987). Two experimental studies and three real-world studies demonstrated measles virus survival between 29 and 120 min, with increasing survival time for lower humidity levels.

Interpretation

Current guidelines for measles contact tracing exposure windows rely on limited research from 1964 to 1987. Additional studies are urgently needed to understand how long the virus is transmissible in real-world settings, particularly given the implications for contact tracing efficiency and resource allocation during outbreak responses.

Funding

U.S. Centers for Disease Control & Prevention (Cooperative Agreement #NU38FT000004).

Research in context.

Evidence before this study

Current US CDC measles contact tracing guidelines recommend that individuals who have occupied shared air within 2 h of an infectious measles patient's exit are considered exposed. According to available sources, this evidence is based on two experimental studies of how long the measles virus remains in the air, three publications reviewing evidence from real-world outbreaks (1985–1987), and a literature review. However, implementation of this guidance varies. In focus group sessions with eleven US health departments, only nine followed the CDC's 2-h guideline while two used a more lenient 1-h post-exit exposure window. Internationally, guidelines also differ substantially: Australia uses a 30-min post-exit exposure window while the United Kingdom focuses primarily on close and face-to-face contacts with exposure of more than 15 min in a “small, confined area.” This variability in contact tracing practices reflects the lack of a standardised, evidence-based definition for measles exposure windows.

Added value of this study

This systematic review provides a comprehensive assessment of available evidence on measles airborne transmissibility to inform contact tracing guidelines. Our findings demonstrate that current guidelines rely on extremely limited, nearly 40-year-old research, with no new primary studies meeting inclusion criteria since 1987. We documented significant evidence gaps, particularly for outdoor transmission and modern ventilation systems, and identified operational challenges from inconsistent health department practices.

Implications of all the available evidence

The outdated and limited evidence base creates substantial uncertainty for health departments, potentially leading to either insufficient contact identification or unnecessary resource allocation. Future research should include laboratory studies under diverse environmental conditions with modern HVAC systems and coordinated observational studies during actual outbreaks. This evidence could refine and standardise exposure guidelines, improving outbreak response efficiency while ensuring adequate public health protection.

Introduction

Measles has re-emerged as an urgent global public health issue, with increasing cases internationally.¹ The ability of epidemiologists to effectively and efficiently identify contacts is essential for stopping transmission.² However, the determination of who is considered a measles contact can vary by jurisdiction due to inconsistencies in exposure window guidelines.

Like SARS-CoV-2,³ measles spreads primarily through airborne and droplet transmission,⁴ and remains infectious in the air for an extended time, even after the infected person is no longer present.⁵ According to the current U.S. Centers for Disease Control and Prevention (CDC) measles exposure guidelines, if a person experiences either of the following without personal protective equipment for any amount of time, they are considered exposed: 1) occupying the same air space at the same time as someone with measles or 2) entering a room within 2 h after a person with measles left.⁵ This paper focuses on evaluating this 2-h post-exit window, assessing its scientific basis and practical implications for contact tracing. According to published measles infection control guidelines,⁵ these exposure guidelines are based on two experimental studies of how long the measles virus remains in the air,^6,7 three publications reviewing the evidence of real-world outbreaks (1985–1987),8, 9, 10 and a literature review.¹¹

Although the US CDC recommends the 2-h post-exit exposure period, implementation may vary within the US. In focus group sessions with eleven US health departments,¹² only nine followed the CDC guideline, while two relied on a more lenient 1-h post-exit exposure window.¹³ Internationally, guidelines vary as well. Australia uses 30 min post-exit as the exposure window¹⁴ while the United Kingdom focuses primarily on close and face-to-face contacts, and exposure of more than 15 min in a ‘small, confined area.”¹⁵ This highlights the variability in contacting practices consistent with lack of a standardised definition for measles exposure windows.

Reducing the post-exit exposure window can substantially reduce the number of people considered exposed and thus reduce the number of contacts requiring notification/quarantine, and the staff hours and costs required to respond.¹⁶ Accurately identifying this timeframe is crucial for ensuring that staff time and funding are appropriately managed and that routine health department work is not disrupted unnecessarily. To ensure that exposure guidelines are based on the strongest data available, we conducted a systematic review of the evidence for how long the measles virus remains transmissible after an infectious case has left an area. For this review, we have defined transmissibility as any part of the complex biological process involved in infection of a person, from a surviving measles virus lingering in the air to formal measles diagnosis provided by a clinician.

Methods

This systematic review examined the duration of measles virus transmissibility in airborne environments to guide epidemiologists in determining exposure periods for contact tracing in public spaces after the presence of an infectious case. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed.^17,18 The protocol was not registered but is available upon request.

Search strategy and selection criteria

The literature search was conducted in PubMed, EMBASE, Web of Science, and SCOPUS for articles published between January 1988 and July 2024. This starting year was chosen as it marks the commercialisation of the taq polymerase enzyme critical for the widespread utilisation of polymerase chain reaction (PCR) technology for accurate measles diagnosis.¹⁹

The review began with a focused search strategy developed with expert input from infectious disease epidemiologists, researchers, and a medical doctor. The preliminary search query applied across all databases was: (“measles”) AND (“exposure period” OR “survival time” OR “environmental stability” OR “airborne infectivity”) AND (“transmission” OR “airborne transmission”). However, this initial search strategy was determined to be too limited.

To ensure a more comprehensive retrieval of relevant literature, the search strategy was subsequently expanded. A broader search query was developed with expert input from a public health informatician utilising MeSH database terms and additional key terms. The final search query was used across all research databases: (“measles” OR “rubeola”) AND (“air microbiology” OR “respiratory aerosols and droplets” OR “airborne” OR “exposure period” OR “survival time” OR “environmental stability” OR “aerosol∗” OR “droplet∗” OR “air”) AND (“disease transmission, infectious” OR “transmission” OR “airborne transmission” OR “survival” OR “mortality”). All results were managed through Covidence²⁰ for de-duplication, title and abstract screening, full-text review, and extraction.

After de-duplication, two independent reviewers screened titles and abstracts to assess eligibility for full-text review. Any disagreements were resolved by a third independent reviewer, who made a final determination. Studies were included if they examined the duration of measles infectivity in the air after an infected person has left a space, specifically in public places (e.g., hospitals, aeroplanes, grocery stores). Studies that occurred indoors and outdoors were both eligible for inclusion. Eligible studies included peer-reviewed articles and grey literature from public health authorities that address contact tracing in relation to exposure periods and review the relevant scientific evidence. Both human and animal studies, as well as laboratory-based investigations on measles infectivity in the air, were considered.

Exclusion criteria included recommendations and studies that were not evidence-based (including mathematical models), unrelated to airborne transmission of measles in public environments, and publications not in English. Studies conducted in non-public settings were excluded because their transmission dynamics differ from those in public spaces. In particular, household transmission studies were not included due to multiple factors, such as prolonged close contact, repeated exposures, and the potential contribution of fomite transmission, which can confound the role of airborne persistence after an infected person vacates a space.

Reviews, editorials, opinion pieces, and duplicate studies or studies with overlapping data sets were also excluded. If an outbreak or airborne transmission event was discussed in both peer-reviewed articles and grey literature, for example, journal publications, only the peer-reviewed articles were included to reduce duplicative reporting of results.

The manuscripts that met the inclusion criteria during the title and abstract screening were advanced to a full-text review. During this phase, two reviewers again independently assessed each manuscript for inclusion. Items that received discordant votes were each discussed during a live meeting attended by both primary reviewers and a third reviewer. In this meeting, each reviewer presented their rationale, and the third reviewer made the final determination after considering all viewpoints. Any manuscripts that passed the full-text review were subsequently included in the data extraction and analysis phase.

Supplemental search strategies

Since no studies were identified in the systematic search, additional search strategies were conducted to ensure comprehensive coverage of the evidence. These three additional methods included 1) review of references from literature reviews identified during the Covidence search, 2) a Google Scholar search utilising a similar search query as was implemented in the study databases, and 3) a review of all literature used by the U.S. CDC to inform the established measles exposure guidelines.⁵ These supplemental search strategies followed the same inclusion and exclusion criteria used during the systematic search, except that there was no limitation applied to the date of publication.

Any literature review references that specified primary data collection of measles exposure data with timeframes were included as part of the supplemental search; this is referred to later in this write-up as the literature review snowball sampling. The literature reviews themselves are still not included in the results, but the primary research articles themselves are included. The second supplemental search strategy was the Google Scholar search, which was conducted using key phrases searched in combination and independently: “Measles Exposure,” “Measles Outbreak,” “Measles Case Study,” “Measles Case”, “Measles Evidence,” “Measles Airborne,” and “Measles Transmission.” Where relevant, authors also reviewed any CDC articles that results from the Google Scholar search. The third strategy involved reviewing the evidence published on the U.S. CDC website.⁵ We consulted with state and local health department partners that followed one- and 2-h exposure windows to verify our systematic review findings and identify any additional sources they may have used. No additional articles were identified through this process.¹²

Data extraction and management

Three staff independently conducted data extraction, and any disagreements were discussed and resolved. Variables included study setting, environmental, and infectious exposure window. Author and year of publication were recorded. All data were documented in a Microsoft Word template.

Statistics

Given the limited data extracted, no formal statistical analysis was conducted.

Quality assessment

The mixed methods appraisal tool (MMAT), a standardised quality assessment tool,²¹ was utilised to measure the reliability and validity of the data identified through the systematic review. All articles included were assessed for the clarity of the research questions, the correctness and quality of the data used to address the research questions, the representativeness of the target population, appropriate measurement of the outcome and exposure, completeness of outcome data, appropriate accounting of confounders in the design and analysis, and proper measurement of the exposure. Quality assessments were conducted independently by two reviewers, blinded, and any disagreements were discussed and resolved.

Role of funders

The funder was not involved in the study design, collection, analysis, and interpretation of results, interpretation of data, writing of the manuscript, or decision to submit for publication.

Results

Data extraction and review

Database searches initially identified only 88 studies across the four databases. Upon expansion of the search terms, 1054 studies from four databases: Scopus (392), Web of Science (262), PubMed (200), and Embase (200), as shown in Fig. 1 below. After removing 495 duplicates, 559 studies remained for the title and abstract screening. After the first round of screening, 463 studies were excluded based on criteria outlined in the methodology, leaving 96 studies for full-text review. Only three articles were excluded for being published in a non-English language. None of these studies ultimately met the previously mentioned criteria for data extraction and analysis.

Fig. 1.

PRISMA Framework with the number of studies identified across each phase.

The supplemental search strategies employed identified only five primary research articles containing evidence that could be used to determine how long measles virus remains infectious in the air after an infected person leaves a space (Table 1). All five articles were published between 1965 and 1987, and two were published with the same lead author. None of the five studies contained primary data that could be used to inform measles contact tracing guidelines for outdoor spaces.

Table 1.

Summary of all publications utilising primary data that could be used to inform exposure windows for measles contact tracing after an infectious case is no longer present in indoor or outdoor space.

Setting	Environmental conditions	Infectious exposure window	Citation
Laboratory experiment	Humidity: 12–15% Temperature: 20 °C	120 min	De Jong et al., 1964a
	Humidity: 68–70% Temperature: 20 °C	<30 min
	Humidity: 36–37% Temperature: 20 °C	>60 min	De Jong et al., 1965b
	Humidity: 60–64% Temperature: 20 °C	30 min
Paediatric outpatient office	Humidity: 22% Temperature: 20 °C	60–75 min	Remington et al., 1985a
	Humidity: not reported Temperature: not reported	∼1hr	Bloch et al., 1985a
Hospital pharmacy	Humidity: not reported Temperature: not reported	29–120 min	Sienko et al., 1987a

^aReferenced in CDC's definition of an infectious exposure window.

^bNot referenced in CDC's definition of an infectious exposure window.

The first two publications assessing measles survival in the air were experimental studies published in 1964⁶ and 1965.⁷ Measles virus survival in the air was used by the authors as a proxy for the window of time a person may be infected after an infected person leaves an indoor space. These studies used aerosolised measles virus in a room where air samples were taken at regular intervals and the surviving virus was estimated by titration (a method used to approximate an estimated concentration of the infectious virus). In both studies, the detections of measles virus in the air under specific conditions at specific time intervals was intended to approximate the potential for a live virus to remain infectious but cannot indicate alone if the remaining virus can infect or replicate within a host. Both articles were published by the primary author J.G. de Jong one year apart. The first study, published in 1964, reported measles survival at 2 h when disseminated in a room with low humidity (relative humidity 12–15%) but less than 30 min in a room with high humidity (relative humidity 68–70%).⁶ Factors such as light or airflow, and temperature were held constant at 20 °C (68 °F). The second article, which was an extension of the first, considered additional factors that may affect the duration of measles infectivity in air, such as additional variations in humidity and the presence or absence of artificial light.⁷ These findings reaffirmed conclusions in the previous article and showed that measles virus survives for less time in the air with increasing levels of humidity when temperature is held constant at 20 °C. In the 1965 study, measles was tested at 36–37% relative humidity (RH) and 60–64% RH, with the virus surviving for at least 60 min at the former RH and only 30 min at the latter RH. The presence or absence of artificial light was not found to have any effect on measles virus survival in the air.⁷

Twenty years later, three publications reported observational data that included evidence on the duration of measles virus transmissibility (infectious cases that resulted in exposed individuals contracting measles with a confirmed diagnosis). In Remington et al. (1985), an outbreak of measles in a paediatrician's office reports three children who developed measles after being exposed in a paediatric office 60–75 min after the index case had departed.¹⁰ According to the timeline provided by the authors, one of the secondary cases arrived at 1:30 PM (60 min after the case departed) and became infected; the next secondary case arrived a little after 1:30 PM (about 65 min after the case departed) and became infected; and the last secondary case arrived around 1:45 PM (about 75 min after the case departed) and became infected. Of the three children that became infected in the 60–75 min after the index case departed, two were under the age of 15 months and one was aged 15 months to 18 years. All three were unvaccinated. A total of 32 children were present within 90 min after the case departed, and only the three children discussed above became infected. An additional 28 children were present in the paediatric office 90 min or more after the infected case departed and none of these individuals developed a measles infection. Interestingly, the observed attack rates for unvaccinated children present with the index case and unvaccinated children that arrived less than 90 min after the index case are similar, around 25%, whereas the attack rate for unvaccinated children arriving 90 min or more after the index case is zero.¹⁰ The second observational publication by Bloch et al. describes a measles outbreak in a paediatric practice where the index patient, who was in the second day of rash, was able to transmit measles to other patients through airborne transmission, even after leaving the office.⁹ The article suggests that measles virus can remain infectious in the air for at least 1 h after the index patient has left the area, providing evidence that measles can be transmitted through airborne spread even in the days after symptom onset, rather than just in the pre-rash period as previously believed. The specific duration and time windows are not reported as they had been in the Remington article; therefore, it is not clear how much greater than 1 h the exposure window was. The final publication summarised how long measles virus remains transmissible after an infectious case leaves a space in a hospital setting. During this outbreak, individuals who arrived 29–120 min after a case was present contracted measles. The minimum exposure window in this outbreak was observed when an 8-month-old was infected with measles after entering an emergency room 29 min after the index case was no longer present. The maximum exposure window observed was when a 24-year-old contracted measles while purchasing medications at the university hospital 2 h after an infected internal medicine resident passed by the pharmacy seeking evaluation for her illness.⁸

The smallest infectious exposure window reported was about 30 min, which was dependent on high humidity (68–70%) and room temperature (20 °C) while the largest exposure window was about 120 min, which was dependent on low humidity (12–15%) at room temperature (20 °C).^6,8 Observational studies reported an exposure window within the range of that reported by the experimental studies, both within the general range of 60–120 min. No additional observational or experimental studies have met the criteria for assessing the exposure window since 1985.

Quality assessment

The 1964 experimental study by De Jong et al. provided the first published evidence that measles persists in the air for an extended period of time. The laboratory experiment presents clear research questions regarding measles virus survival in relation to relative humidity and seasonal morbidity patterns. The titration method used in the study is intended to estimate the concentration of infectious virus, which was appropriate given the research questions explored. While the experimental measurements of virus survival appear accurate, the study fails to account for important confounders such as temperature, airflow, and ventilation. Notably, the research does not address whether the surviving virus remains truly infectious in humans or examine specific exposure windows, which would be crucial for understanding transmission dynamics. In a modern laboratory study, researchers may have been expected to demonstrate if the virus was capable of replication through methods such as viability-PCR or inoculation of susceptible cell lines.

De Jong et al.,'s 1965 follow-up experimental study expands their previous work by including temperature and artificial light as variables alongside relative humidity. The measurements are appropriately designed with clear time points and exposures. The titration method was used again in this study and remained appropriate. While the study accounts for some potential confounders by individually exploring temperature and light effects, the data for 15 °C is not reported and therefore excluded, and the authors still neglect airflow and ventilation factors. As with the previous study, the absence of human participants limits translation to real-world transmission scenarios and the researchers did not include any methodology to demonstrate if the virus had the capacity to replicate or infect a host.

The 1985 study by Remington et al. reported on a measles outbreak in a Muskegon, Michigan paediatrician's office primarily affecting children and effectively documents the first evidence of measles virus transmissibility after a case leaves a space in a real-world setting. The research did this by tracking interactions between index cases and contacts under controlled environmental conditions (22% humidity, 20 °C) and categorised exposure windows clearly (<90 min versus ≥90 min after index case departure), providing valuable data on transmission timing. The study appropriately accounts for temperature, humidity, and airflow confounders. However, the applicability of findings considering recent modernisation of ventilation systems remains a concern.

Bloch et al., 's 1985 investigation into an outbreak in a DeKalb County, Georgia paediatrician's office also provides compelling evidence for measles virus survival in the air for at least 1 h. Through meticulous documentation of exposure windows using appointment schedules, sign-in logs, and participant interviews, the researchers established that transmission occurred to a patient who entered a paediatric office 1 h after the source patient had left. The study's strength lies in its thorough airflow analysis, demonstrating that the virus spread through the ventilation system throughout the facility. The researchers effectively ruled out fomite transmission and documented airflow dynamics, though humidity and temperature data were not captured.

The 1987 study by Sienko et al. clearly examines the duration of transmissible measles virus in a Michigan university medical setting. The research effectively documents exposure timeframes, enabling assessment of how long measles remains transmissible after a source patient departs. However, the study fails to account for potentially significant confounders including temperature, humidity, and ventilation characteristics.

Discussion

The five studies identified present valuable but limited evidence regarding measles virus transmission and survival, with methodological constraints that affect their applicability to contemporary settings. The De Jong et al. studies (1964, 1965) provide important foundational laboratory data but fail to address human infectiousness or account for all relevant environmental variables. The observational investigations from the 1980s offer useful real-world transmission insights through careful exposure tracking, yet were conducted when ventilation systems differed considerably from modern systems—lacking advanced filtration, air exchange capabilities, and standards now common in healthcare facilities.^22,23 Inconsistent measurement of environmental factors, such as temperature, humidity, and airflow, is evident across the observational studies. Furthermore, only Remington et al. (1985) presented attack rates in a real-world setting that could inform exposure window guidelines. While these articles collectively support the persistence of measles in the air between 29 and 120 min, their age and methodological limitations suggest additional research is needed to more accurately determine the risk at specific time intervals.

Study limitations

Search criteria in the systematic review were limited to manuscripts published in English, potentially excluding useful research in other languages. The initial search was limited to January 1988 to July 2024 after PCR testing became widespread to reduce potential bias from improper diagnosis, but this excluded earlier studies that provided valuable insights into understanding the exposure window. However, our supplemental search mitigated this risk by including earlier studies. Also, changes in Medical Subject Heading (MeSH) terms over time may have resulted in the omission of relevant studies that were indexed under our search terminologies. Additionally, certain older papers lack abstracts or are available only as scanned PDFs without searchable text, which would not have been detected by the search engines utilised and would have led these papers to be inadvertently excluded. Furthermore, while we developed a comprehensive set of search terms, it is possible that some relevant studies were missed due to variability in terminology used by authors. Finally, our supplemental search strategy researching publications used to inform national guidelines was limited to those publications referenced by the US CDC, as this research was undertaken in response to needs identified by US health agencies. Some aspects of this limitation were likely addressed through the initial global systematic search and the global Google Scholar search.

Research needs

Laboratory studies are needed to better understand how the measles virus survives in the air and on surfaces under different environmental conditions and in various settings. Current literature only evaluates measles survival at 20 °C (68 °F) and fails to consider the diverse factors present in public settings. Additionally, existing studies have only examined measles survival in controlled environments. This limited perspective only addresses a single aspect of the chain of transmission involved in an outbreak. Future research should investigate how measles survives in real-world environments such as classrooms, open-concept offices, grocery stores, restaurants, aeroplanes, and hospitals using modern environmental sampling techniques. These studies should incorporate lessons learnt from COVID-19^24,25 and maximise advances in laboratory science, such as PCR.²⁶ Revisiting the foundational science of measles transmission with these modern tools will be essential for understanding the real-world studies as well as informing the accurate parameters for modern modelling studies.

Observational studies may help understand exposure windows considering modern air filtration. The U.S. CDC has published recommendations for air changes per hour for removal of airborne contaminants that can be further integrated into understanding the spread of airborne diseases.²⁷ However, additional research is needed as these recommendations are not specific to measles and are solely focused on occupational settings (i.e., healthcare facilities). To shed light on this, information on the duration, location, and timing of exposure, as well as the number of cases that have and have not been attributed to a specific source of an exposure would be beneficial. This information could be gathered through coordinated efforts by state and local health departments conducting measles case investigations, as well as hospital systems and healthcare clinics. Agencies with available resources may consider collecting exposure time windows for each contact identified in a case in a database. Coordinating entities could pool de-identified data on the exposure windows for each contact and calculate the attack rates for exposure times. The impact of this data will likely be reliant on standardised data collection across jurisdictions, which may require significant coordination and financial investment.

This systematic review also identified a gap in evidence for contact tracing in outdoor public spaces. Although this systematic review was not limited to indoor settings, the researchers did not identify any experimental or observational studies that assessed measles virus transmissibility in outdoor air, indicating a need for research.

Public health implications

This systematic review highlights that current evidence is limited and outdated, and does not account for modern air filtration, as well as transmission patterns in various settings such as schools, work, and outdoor settings. Advancing research to clarify risk for the measles exposure window is critical to ensure the right balance between prevention of missed exposures and secondary outbreaks with the responsibility of public health professionals to avoid unnecessary quarantines, staff overtime, and tax-payer costs. Such research would improve public health responses' efficiency and effectiveness, fostering trust in evidence-based recommendations, and may be used to inform updated, evidence-based guidelines globally.

Contributors

H. Wellham is the corresponding author, and led the systematic review and writing; C. Rivers conceived and supervised the review, and obtained the funding; S. Soneja supervised the writing and editing; E. Toner and S. Ravi contributed expert opinion with the selection of MeSH search terminology, selection of databases, and summary of evidence; S. Gillani coordinated all efforts and experts to develop and revise the search terms; S. Gillani, F. Nkempu Ameaka contributed significantly to the study screening, analysis, and writing; H. Wellham and E. Campbell contributed to the quality assessment, writing, and editing. All authors had access to the data. H. Wellham, S. Gillani, and F. Nkempu Ameaka directly accessed and verified the underlying data in this report. All authors read and approved the final version of the manuscript.

Data sharing statement

The data collected for this analysis are available upon reasonable request.

Declaration of interests

CORI is one of 13 InsightNet centres funded by the U.S. Centers for Disease Control & Prevention Center for Forecasting and Analytics (Cooperative Agreement #NU38FT000004). C Rivers is a published author that has received payments for writing on related topics. S. Soneja is a Co-I on a Johns Hopkins University Nexus grant, which focuses on convening stakeholders related to the use of modelling and analytics in outbreak response, has received honoraria from NASA for serving on a grant review panel related to air quality and health in 2024 and 2021, and attended the InsightNet conference funded by the InsightNet cooperative agreement n 2025. The views expressed in this article do not necessarily represent the views of CDC, InsightNet, NASA, or Johns Hopkins University.