COVID-19 herd immunity: 7 questions, answered

Last updated on Dec. 7, 2020

As the number of confirmed coronavirus (COVID-19) cases continues to rise worldwide, there’s been a lot of talk about herd immunity and how it might change the course of the pandemic.

But what is herd immunity, exactly, and how is it achieved? Once we have it, will it diminish the need for masks and social distancing, or allow people to return to their pre-pandemic lifestyles?

We spoke with our infectious diseases and infection control specialist Roy Chemaly, M.D., for details.

What is herd immunity?

Herd immunity happens when enough people have developed immunity to a particular infectious disease that the risk of further community transmission is either eliminated or significantly reduced.

This requires a certain percentage of the population to have been exposed and infected with the disease — in this case, COVID-19 — so that they develop immunity against it in the form of antibodies. These antibodies not only protect the person who has them from re-infection for a certain period of time, they also prevent them from passing along the disease to others, because their immune systems kill whatever it is that causes it — a virus or bacteria — when they encounter it again.

What does it take to achieve herd immunity?

That varies by disease. For this particular coronavirus, doctors estimate that about 40% to 50% of the human population would need to have antibodies in order to have herd immunity as a species.

What is that COVID-19 estimate based on?

There’s actually a formula used to calculate that percentage (herd immunity threshold = 1-1/R0). This formula includes the basic reproduction number or “R-zero,” which indicates the average number of new people that a single infected individual can expose and infect. That figure for the flu is around 1.3, meaning that each infected person infects an average of 1.3 more people. Unfortunately, for the coronavirus, it’s between 2 and 3. So, this is a very contagious disease.

When will we achieve herd immunity against this coronavirus?

When we have a vaccine for COVID-19. Because we can’t just rely on natural immunity, or the kind that develops after you’ve been infected and recovered.

We probably need around 70% of the population to have developed antibodies in order to halt community transmission of COVID-19. But we're not even close to that yet.

What happens when we do reach herd immunity for COVID-19?

That depends. There have been some indications that this may be like the flu, and that immunity may not last longer than four or five months. So, immunity may wane over time, and people would need to be revaccinated. But nobody really knows yet.

It is rare for someone to be infected with the coronavirus twice. But viruses are known to change over time, or mutate. So, the bottom line is just because you’re immune today doesn’t mean you’ll still be immune 3 or 6 months from now.

Can you build herd immunity by throwing a COVID party?

No. This is the absolute worst thing to do. Because many people will get infected and transmit it to others in the community. And you can’t predict which ones will develop severe symptoms or require hospitalization, and maybe even die.

In some instances, we’ve heard about 50 or 60 people being infected at just one of these parties. And some party-goers may have been infected, but not show any symptoms. So, afterwards, they’re out transmitting the virus to their friends and family, because they’ve mistakenly assumed they didn’t get it.

Also, we still aren’t sure how long immunity might last, so even if party-goers did get infected and recover, after 3 or 6 months, they might not even be protected anymore. Then it was all for nothing.

What will happen once we have a COVID-19 vaccine?

Once we have one, it will still have to be distributed, and then people will have to actually get the vaccine. Then, it will take them a few weeks to build up enough antibodies to be protected.

Also, just because you have a vaccine doesn't mean the disease is no longer a threat. Think about the flu vaccine. People still have to get that every year to be protected, and even then, it’s estimated to only be between 30 and 80% effective against whichever strains are circulating that season. So, a vaccine will help, but it may not completely neutralize the threat. 

It’s possible that herd immunity could be achieved for a certain period of time, and then wane. Once we learn more about how long immunity from a vaccine lasts, we’ll be in a better position to determine if we need to re-vaccinate people in order to maintain herd immunity.

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