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A single person may build immunity to a disease, but to control the spread of that disease throughout the population, we must develop what’s known as “herd immunity.”

If a certain percentage of a group of people — the so-called herd — is immune to a virus, then an individual in this group is less likely to become infected because the virus has a tough time transmitting through the herd. We develop herd immunity by naturally building our antibodies to a disease or, more commonly, by having them bolstered by vaccines.

“Stop transmission and you stop the virus,” explains infectious-disease researcher Daniel Altmann, PhD, a professor of immunology at London’s Imperial College.

In an open letter to policymakers worldwide published on the website of the medical journal Lancet, Altmann and colleagues explained the concept of herd immunity as it relates to COVID-19 and outlined the essential goals the world needs to reach before ending social distancing.

“How can one determine how much herd immunity is sufficient to mitigate subsequent substantial outbreaks of COVID-19? . . . [T]he herd-immunity calculation suggests that at least 60 percent of the population would need to have protective immunity, either from natural infection or vaccination,” they write. They go on to explain, “The linchpin for a strategy to move out of lockdown seemingly rests on increased testing and contact tracing, possible return-to-work permits based on immune status, repurposed or new therapeutics, and, finally, vaccination.”

To explain how herd immunity works, we spoke with Altmann:

Experience Life | What is herd immunity, and why is it so important?

Daniel Altmann | Herd immunity is a mathematically based concept that comes largely from vaccine studies. It relates the infectivity of a pathogen — the “reproduction rate” of a disease, termed R0 and pronounced “R-naught” — to how large a proportion of the population would need effective immunity to give effective coverage and prevent that pathogen from spreading. If we could reach the threshold for herd immunity in this case, we might be relatively “safe” from repeat pandemics. If calculations of R0 for COVID-19 of about 2.4 are correct (that is, each infected person on average infects 2.4 further people), then we need herd immunity at greater than 60 percent.

Herd immunity as a concept is most commonly applied to vaccine coverage. So, if you have a disease like smallpox, where humans are the only host and we’ve had decades of effective, high-coverage herd immunity through vaccination, there’s nowhere for the microbe to hide and it gets eliminated.

EL | What is “shedding” as it relates to herd immunity?

DA | Anyone carrying the live virus, whether or not they feel unwell, can be shedding it and spreading to others. The period of live-virus shedding generally precedes the time when antibodies start to be made.

EL | How might we reach herd immunity in the instance of COVID-19?

DA | As there is no vaccine yet, herd immunity can only be through natural exposure, whether symptomatic or asymptomatic. Many studies in different countries are starting to assess current levels of herd immunity by antibody testing. Few places are coming up with levels above 10 percent. So, the only way to reach 60 percent or greater, without risking many more deaths, is to have effective vaccines.

EL | If we know that building herd immunity has been an effective strategy for mitigating other diseases, why does there seem to be hesitance with COVID-19?

DA | Because we normally think of herd immunity in terms of vaccination programs. It’s rare in modern times that we allow a lethal infection to pass through our community unchecked, with natural herd immunity as our best mitigation strategy.

EL | Are there examples of herd immunity anywhere so far when it comes to COVID-19?

DA | The anecdotal evidence from Sweden sounds as if the level of herd immunity in some parts of the population may perhaps be double the figures from other affected countries, yet still far too low to offer effective herd immunity and protection against a subsequent wave.

EL | Do we know definitively that people actually build lasting immunity once they’ve had COVID?

DA | It’s very early days and data are still coming in. We know that infected people make antibody, and a component of this antibody has the ability to neutralize the virus effectively. However, most data so far are from hospitalized people — not the vast majority of exposed people in the community, who may have had less exposure to virus, so made lower amounts of antibody. Some exposed people who have been studied have shown virtually no antibody.

Also, we know little about the true correlates of protection in this disease: The immune system is complex and multifaceted, and antibody may not be the key marker we should be measuring. That is, there are many potentially protective components of immunity beyond just the level of antibody. For some infections, a better correlate of protection seems to be the frequency of particular white blood cells in the blood (T cells) that can kill virus-infected cells. In the case of the related infections SARS and MERS, T-cell immunity seemed to be far more durable than antibodies.

This is very much a global problem — we all face the same threat from the same virus. So, we need to learn from global efforts in state-of-the-art, evidence-based science and pull together to learn the best answers as rapidly as possible. This can’t be achieved with local solutions.

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