Herd Immunity For COVID-19 (GAO-20-646SP)

Summary

A GAO Science & Tech Spotlight on herd immunity for COVID-19, GAO-20-646SP, dated July 2020, from GAO's Science, Technology Assessment, and Analytics team. It explains that a population can establish herd immunity once a large enough portion, typically 70 to 90 percent, develops immunity through infection or vaccination. It states that for diseases with no vaccine, herd immunity can develop through exposure to and recovery from the disease. It cautions that achieving herd immunity to COVID-19 without a vaccine could result in significant morbidity and mortality rates, and that limited information about the disease makes its implications hard to understand.

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Science, Technology Assessment,
JULY 2020
and Analytics
WHY THIS MATTERS
Increasing the immunity of a population to an infectious
disease like COVID-19 can slow the spread of
SCIENCE & TECH SPOTLIGHT: infection and protect those most vulnerable. However,

HERD IMMUNITY FOR
with limited information about important aspects of
COVID-19, there are challenges to understanding the
implications of herd immunity in the current pandemic.
COVID-19

/// THE SCIENCE For diseases where no vaccination is available, it is possible to develop
herd immunity through exposure to, and recovery from, the disease.
What is it? A population can establish herd immunity to an infectious However, if COVID-19 runs its natural course, this approach would
disease once a large enough portion of the population—typically 70 to entail the risk of severe disease or death. Given the risk associated with
90 percent—develops immunity. Reaching this “herd immunity threshold” COVID-19 infections, achieving herd immunity without a vaccine could
limits the likelihood that a non-immune person will be infected. In general, result in significant morbidity and mortality rates.
immunity develops through either infection (resulting in natural immunity)
or vaccination (resulting in vaccine-induced immunity). Herd immunity How mature is it? Knowledge of previous infectious disease outbreaks
helps protect people not immune to a disease by reducing their chances where a vaccine was available has allowed researchers to identify how
of interacting with an infected individual. This process slows or stops the herd immunity was achieved for those diseases. However, researchers
spread of the disease. currently have insufficient data on the factors that could contribute to herd
immunity for the COVID-19 pandemic. These factors include the herd
immunity threshold, the number of secondary cases typically generated
by an infected individual, the viral mutation rate, and the length of time
immunity lasts.

At this stage in the COVID-19 pandemic, researchers have insufficient
data to draw definitive conclusions about the level of immunity conferred
by an infection, or how long immunity to the disease might last. For
example, in order to determine the herd immunity threshold, it is important
to know how contagious the disease is–which is affected by factors such
as how many susceptible people an infected person can infect. While
researchers have developed estimates for how contagious COVID-19 is,
uncertainties about case reporting and testing—such as uncertainty in
the accuracy of some tests—make this calculation difficult. Some peer-
reviewed research on COVID-19 suggests the average number of people
infected by a contagious person ranges from about one to seven.
Figure 1. Herd immunity helps reduce the likelihood of disease transmission from infected
individuals to non-immune individuals.

How does it work? Once a community has established herd immunity,
someone without immunity is less likely to be exposed to an infectious
individual during an outbreak. For example, because there are more
people with immunity in the population, there are fewer people susceptible
to infection, and thus the number of potential transmissions is limited.
Similarly, those who are immune will not be infected, and thus will not
transmit the disease to others. Both of these situations help limit the size
of the outbreak.

If an effective vaccine is available for a virus, achieving herd immunity
can require a high rate of vaccination in the community. For diseases that
spread more easily, more people must have vaccine-induced or natural
immunity to achieve herd immunity. However, if a virus mutates quickly,
the community’s herd immunity may be relatively short-lived because the
immunity from prior infection or vaccination may no longer be effective.
Also, the disease can still circulate in segments of the population that are Figure 2. While data on previous disease outbreaks are available, for COVID-19, we don’t
not immune, such as those with weakened immune systems who cannot yet have all of the necessary data for many of the relevant factors.
effectively form immunity.

GAO-20-646SP Herd Immunity for COVID-19
Science, Technology Assessment,
and Analytics

While analyses of viruses related to the novel coronavirus have shown ■ Inconsistent immunity. Even if herd immunity is eventually
that infection can provide some level of immunity, such immunity did not achieved, outbreaks may still occur because immunity may not be
appear to last longer than a year. Studies of other infectious diseases, uniform across the general population.
such as polio, exhibit a range in the threshold for herd immunity, the
average number of people infected by an infected person, and how long /// POLICY CONTEXT AND QUESTIONS
people remained immune, among other factors.
■ What studies are needed to gather data on the parameters for
/// OPPORTUNITIES herd immunity against COVID-19, and how can this be reflected in
resource priorities?
■ Halt disease spread. Achieving herd immunity will slow or stop the
spread of a disease within a population and limit the size of outbreaks. ■ If a COVID-19 vaccine is approved but the number of available
doses is still limited, what factors may need to be considered in
■ Support economic recovery. Established herd immunity could help deciding who should be vaccinated first?
bolster the economy by allowing people to safely return to work and
conduct other activities. ■ If the path to COVID-19 herd immunity is uncertain, what steps
might be needed to help strike a balance between public health and
■ Restore medical capacity. Established herd immunity could help
enabling the economy and society to function smoothly?
ease the burden on the medical system as fewer patients seek
treatment. ■ How could infectious disease modeling assist in working towards
establishing herd immunity?
/// CHALLENGES
/// SELECTED GAO WORK
■ Limited data on immunity. Currently, there are a number of
unknown factors, such as whether a COVID-19 infection leads to GAO has previously issued the following products related to COVID-19:
immunity and how long immunity might last. It may be necessary for
more time to pass to monitor individuals who have been infected and ■ Science & Tech Spotlight: Coronaviruses, GAO-20-472SP
recovered and thus determine how long they show disease immunity.
■ Science & Tech Spotlight: COVID-19 Testing, GAO-20-584SP
This information is needed to determine the herd immunity threshold.
■ Science & Tech Spotlight: COVID-19 Vaccine Development,
■ Implications of natural herd immunity. Relying on natural
GAO-20-583SP
immunity from the disease progression of COVID-19 could allow a
population to establish herd immunity. However, such an approach ■ Science & Tech Spotlight: COVID-19 Modeling, GAO-20-582SP
risks exposing people to a debilitating and potentially fatal disease. ■ Infectious Disease Modeling: Opportunities to Improve Coordination
■ Testing limitations. If infection confers immunity, researchers and Ensure Reproducibility GAO-20-372
need accurate data on the number of recovered individuals and
other disease transmission parameters to determine if or when /// SELECTED REFERENCES
herd immunity is achieved. Accurate testing data are critical to
Anderson, R. M., and May, R. M. “Vaccination and herd immunity to infectious diseases.”
understanding these parameters. However, there are still challenges
Nature, vol. 318 (1985): pp. 323-329.
associated with the use and availability of antibody tests, as well as
with determining their accuracy. Randolph, H. E., and Barreiro, L. B. “Herd Immunity: Understanding COVID-19.” Immunity,
■ Lack of vaccine. A vaccine could help a population safely achieve vol. 52, no. 5 (2020): pp. 737-741.
herd immunity. However, developing a vaccine is a complicated
Galanti, M, and Shaman, J. “Direct observation of repeated infections with endemic
process that is costly, typically requires 10 to 15 years or more, and
coronaviruses.” Forthcoming. Published on medRxiv.
many candidates fail during the development process. Efforts are
underway, however, to accelerate the process for COVID-19.

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GAO-20-646SP Herd Immunity for COVID-19

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