GAO-20-646SP 1 (2020-07-07)

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                            Science, Technology Assessment,
G       A      O         Iand Analytics




What is it? A population can establish herd immunity to an infectious
disease once a large enough portion of the population-typically 70 to
90 percent-develops immunity. Reaching this herd immunity threshold
limits the likelihood that a non-immune person will be infected. In general,
immunity develops through either infection (resulting in natural immunity)
or vaccination (resulting in vaccine-induced immunity). Herd immunity
helps protect people not immune to a disease by reducing their chances
of interacting with an infected individual. This process slows or stops the
spread of the disease.



                 No hed imunit

Susceptible  * Infected

Immune    -      Disease transmission

Source: GAO adaptation of NIH graphic. I GAO-20-646SP

        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
not immune, such as those with weakened immune systems who cannot
effectively form immunity.

For diseases where no vaccination is available, it is possible to develop
herd immunity through exposure to, and recovery from, the disease.
However, if COVID-19 runs its natural course, this approach would
entail the risk of severe disease or death. Given the risk associated with
COVID-19 infections, achieving herd immunity without a vaccine could
result in significant morbidity and mortality rates.

How mature is it? Knowledge of previous infectious disease outbreaks
where a vaccine was available has allowed researchers to identify how
herd immunity was achieved for those diseases. However, researchers
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.

            Percentage of
            the population
            who must be
            immune to reach
 Disease    herd immunity
COVID-19       Unknown

   The average
 number of people
   infected by a
contagious person
  t 5-7

Estimated duration
   of immunity
     -18 years
     -3-5 years

11-18        Lifelong
14         -15-20 years

     Source: GAO adaptation of Johns Hopkins University School of Hygiene and Public Health,
     Health Department of Catalonia (column 1), Johns Hopkins University School of Hygiene and
     Public Health, Health Department of Catalonia, Umea University (column 2), and University of
     Auckland, and the Centers for Disease Control and Prevention, (column 3). I GAO-20-646SP

        While data on previous disease outbreaks are available, for COVID-19, we don't
yet have all of the necessary data for many of the relevant factors.

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

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