What is Herd Immunity?
Imagine there’s a fire in the middle of a big forest. At first, you think, “Forest fires are normal! They happen every year. It’ll burn itself out soon enough.” Then, the fire doesn’t go out. In fact, it spreads. And spreads. And soon, so much of the forest is on fire that you have no chance of putting it out by yourself. There are a few things you can do:
(Disclaimer: I don’t know much about forest fires, so please bear with me for the sake of this metaphor.)
You can call in the fire brigade to throw water on the fire. As water is being sprayed, the fire will continue to spread in other places. It might take awhile to completely put out the fire, which means the fire will spread more before it finally stops. Eventually, a lot of trees will be burned, and probably won’t light on fire again.
You can cut away the trees from the forest surrounding the fire to create a space between the fire and the surrounding forest. This will prevent a lot of the fire from spreading outwards, since the fire cannot easily jump across the empty space. This only works, however, if masks are worn - oops, I mean trees are cut down - on all sides of the fire. There also still a chance the fire could still spread anyway, like if a strong wind carried it across the empty space.
You can spray water on all of the unburned trees, to make them much, much harder to light on fire. Once the fire is gone, the unburned trees will be absolutely fine.
In this metaphor, the fire is COVID-19. The water is our already-overburdened hospital system and medical technology. Cutting down trees is like wearing masks and social distancing. Putting water on the trees in the rest of the forest is like vaccinating the rest of the healthy population. The best thing we can do is all three: keep our hospital systems working as well as possible, wearing masks and staying home, and vaccinating as many people as possible.
At the start of the pandemic, the virus spread very quickly. A person might catch the virus and spread it to their cashier at a grocery store, the elderly woman behind him at line, and Uber driver on the way home. That person might not even show symptoms for another day or two. The fire is spreading quickly.
If that person had stayed home or wore a mask, the virus would not have spread nearly as much on that day. The fire is less able to jump across the empty space.
However, what if some of the grocery store cashier, elderly woman, and Uber driver were already immune to the virus? They would never even get sick. These trees are already sprayed with water, so they won’t burn even if the fire makes it to them.
If some people are immune to the virus, then there is a lower risk of catching it - and spreading it - when you go out. Your classmate is less likely to be contagious and the grocery store clerk is less likely to get sick after exposure. If enough people are immune to the virus, it will stop spreading at all! When enough people are immune, the virus can’t spread because there aren’t enough vulnerable people around to spread it to. This is called herd immunity. Once we reach herd immunity, COVID-19 will stop spreading (or at least slow down a lot) and maybe even disappear. We don’t know exactly how many people need immunity to reach herd immunity. For measles, it means 95% of people, and for polio, it means 80% of people. For COVID-19, that number might be closer to 70%, but we don’t know for sure.
There are two ways to get immunity. The first is to catch COVID-19 and recover. In fighting off the virus, your body develops the ability to fight it off again, immediately, without getting sick again! The other way is from a vaccine, which allows your body to develop that defensive ability without ever getting sick in the first place. [For more about how immunity and mRNA vaccines work, read “How Does an mRNA Vaccine Work?]
Why don’t we skip the vaccine and let immunity spread “naturally”, by allowing everyone to get COVID-19? That would be like letting the forest burn. Eventually the fire would stop, but not until a lot of trees had been damaged. As of December 20, 2020, there have been 17.7 million recorded cases of COVID-19 in the US. There have been 317,000 COVID-related deaths. If everyone in the US got sick with COVID-19 and died at this same rate, nearly 6 million Americans would die. (These numbers are very rough. There are other factors to consider, such as asymptomatic cases and cases that go unreported. There would also be deaths due to unrelated causes that would have been treatable if hospitals were not overwhelmed. Suffice it to say, the death toll would be significant.)
If we can reach herd immunity from widespread vaccinations, we can save millions of lives in the US alone. We could reach herd immunity without having everyone (or nearly everyone) in the US get sick. The biggest limitation right now is production and distribution of the vaccine. It might take months to reach herd immunity. We might not know whether people keep this immunity over months and years. It seems most likely that we will reach herd immunity sometime next summer, and that we have a good shot at knocking out the virus by the end of 2021. In the mean time, wear masks and stay home, so we can save as many lives as possible until then.