“Herd immunity” can happen once a large part of a population—say 70-90%—develops immunity to a disease. This generally happens by infection (conferring natural immunity) or vaccination. Herd immunity can help slow or stop a disease’s spread, but many people may die before it happens.
It is currently unclear how herd immunity could happen for COVID-19. For example, the data are insufficient to show how much immunity infection confers, or whether it’s enough to prevent re-infection. How long any immunity will last is also unknown. Related viruses confer some immunity after infection, but that immunity doesn’t seem to last longer than a year.
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.
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.
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.
While analyses of viruses related to the novel coronavirus have shown that infection can provide some level of immunity, such immunity did not appear to last longer than a year. Studies of other infectious diseases, 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 people remained immune, among other factors.
