Everything You Need to Know About Herd Immunity

It’s the ultimate goal, but attaining it can be difficult (and deadly)

Health care professionals test for Covid-19 at the ProHEALTH testing site in Jericho, New York on March 24, 2020. Photo: Newsday LLC/Getty Images

By now you’ve heard lots of talk about herd immunity to Covid-19 — perhaps the hope that it will be key to getting life back to some semblance of normalcy. Or, maybe you’ve heard striving for herd immunity “would be a disaster” that causes millions of deaths. So which is it?

It’s actually both. For that to make sense, you need to understand what herd immunity is, how a population reaches it, and the risks of getting there only through infections.

What is herd immunity?

The basic concept of herd immunity is pretty straightforward: The more people in a population who are immune to a disease, the harder it is for that disease to spread through the community. For example, if none of the 1,000 residents in a small town were immune to measles and someone sick with measles visited, about 900 townspeople would likely end up with measles — the disease would rage through the town like wildfire.

But if 93% to 95% of the town were immune to measles, either because they had it as children or because they had been vaccinated, possibly no one would get sick. Even if some of the 50–70 nonimmune people had direct contact with the infectious visitor, the town wouldn’t see an outbreak: The high proportion of immune townspeople would protect most of those 50–70 vulnerable folks. How do they do that? They basically insulate the unprotected people. Viruses seek hosts to allow them to replicate and then find the next host. If all the virus encounters are immune people, it has nowhere to go. As long as the nonimmune people are randomly spread throughout the town, the virus has a lower chance of finding them. Even if it finds a couple of them, they won’t be able to pass it on to many others if they’re mostly surrounded by immune people. The virus can’t reach everyone if not enough people can become infected to keep passing along the disease. (That said, if those 50–70 people all attend the same house of worship and the infected visitor visits that service, chances are there will be an outbreak affecting most of the nonimmune people in town — we’ll get to that.)