A: The term “herd immunity,” also sometimes called community immunity, refers to a situation where a lot of people in a population are not susceptible to a virus–either because they have already had it and are unlikely to get it again, or because they’ve been vaccinated.
That is to say, their immune systems have been primed to respond to the virus, and they won’t get infected with it when they are exposed again. All those people with individual-level immunity mean that the virus has a hard time finding a host that it can infect, replicate, and spread to a new host.
The situation where so many people are immune that a virus is unlikely to find a new host is known as herd immunity.
With herd immunity, even though *some* of the people in a population are susceptible to the disease, they’re unlikely to be infected simply because they probably won’t encounter the virus circulating in the population in the first place. Herd immunity is the goal of having as many people as possible get a vaccine. Having a lot of people vaccinated helps to protect even those who cannot or will not be vaccinated.
Herd immunity does not directly protect individual people from infection. It just limits how fast a virus can spread through a population.
One of the reasons that COVID-19 is spreading so rapidly around the world is that there are literally billions of people who are susceptible. Up until just a few months ago, *no one* had ever had it. That means everyone was susceptible and the virus had no trouble at all finding new hosts.
What would it take to get to herd immunity for COVID-19? Roughly speaking scientists think something like 70-80% of the population would need to have immunity to produce herd immunity.
How do we get to herd immunity? There are two routes, and they both have major problems in terms of being the magic bullet that stops the pandemic right now.
1. A vaccine.
The safest path to herd immunity (and maybe the only way) is to develop a vaccine, so that we can become immune without ever having to be infected. A vaccine works by giving your immune system clues as to how to identify the virus before you encounter it naturally. This allows your body to mount a speedy and effective immune response later on when someone sneezes on you, and you don’t end up getting sick. But a vaccine is a long ways off–at least 18 months, and maybe much longer. And even after a vaccine is approved, it will take years before we have enough people covered to really produce herd immunity. Staying home until a vaccine becomes available isn’t a realistic option.
2. “Natural” herd immunity.
Okay so a vaccine is too far off. What if we just let everyone get COVID-19? As epidemiologist Gideon Meyerowitz-Katz says, “…this is simply nonsense. Herd immunity without a vaccine is by definition not a preventative measure.” If we just let everyone get the disease, then everyone is sick. That’s not stopping the spread of the disease–it’s encouraging it! Arguing for this let-it-burn approach to achieving herd immunity would require hundreds of millions of people to fall ill all at once. If we guess conservatively at a 2% mortality rate, we’re talking about over 4 million deaths in the United States alone! YIKES. Plus, we are still learning about the long-term health consequences of COVID-19 infection for survivors. Letting (or even encouraging) everyone to just catch the disease comes with terrible consequences.
Not only that, we don’t yet know if recovering from COVID-19 means you are immune to it in the future. So even if we could accept the consequences of allowing millions of people to get sick and even die, we don’t know if that would produce herd immunity. In fact, rapid widespread infection could provide the virus just the opportunity it needs to mutate more rapidly–which could undermine herd immunity!
Science Alert Op/Ed on Herd Immunity
Edited to add: So what do we do then?? Outbreak control measures: isolation and contact tracing for confirmed cases, testing all the people who have a reasonable suspicion of exposure (yes even asymptomatic people), continued improvements to treatment options, and a gradual, thoughtful, systematic roll-back of the social distancing measures.
