When we get to 70% vaccinated, this will instantly be over–right?

Infection and Spread

A: Not necessarily. We need to dig in a little to the difference between reaching the herd immunity *threshold* and the end goal–herd immunity.

The upshot: herd immunity is the ultimate goal of a vaccination program, and reaching the herd immunity threshold is the first step toward it. Also, patterns of vaccination matter. Protecting people who are likely to get severe COVID-19 due to their age or health conditions is (very) important. But it’s also important for the people at highest risk for *transmitting* COVID-19 to get vaccinated. Looking at you, young adults! 👀 This helps protect those who can’t be vaccinated, or the rare person whose vaccine just doesn’t work.

Herd immunity: when we get to a point where there are no more outbreaks because just about everyone is vaccinated. COVID-19 becomes rare, and even people who can’t get vaccinated for some reason are unlikely to ever be exposed.

Herd immunity threshold: science’s best guess at the number of people who need to be vaccinated before we can think about getting to herd immunity. Reaching the threshold is the first step to reaching herd immunity–but not the end game.

🐮 Herd immunity for COVID-19 (or any other disease that is contagious from person-to-person) happens after many people have immunity to an infectious disease. So many, in fact, that even if one person happens to get it somehow, they don’t spread it to other people because just about everyone they come into contact with is immune. One case never becomes two cases, which never becomes 5 cases, and so on–herd immunity protects us all from outbreaks.

One of the hallmarks of reaching the state of herd immunity is that there are just about no cases of the disease. Even people who cannot be vaccinated at all–people who are allergic to the ingredients in the vaccine for example–will never get COVID-19 because they’ll simply never be exposed to it. The disease does not circulate.

📈That is not the same thing as the herd immunity *threshold*. The herd immunity threshold is our best guess at the number of people who have to be immune in order to eventually reach herd immunity. This threshold is thought to be around 8 out of 10 people who have to be immune to COVID-19. It’s specific to each disease, and the threshold is influenced by lots of factors like how a disease spreads, how infectious it is, how much contact people have with one another, whether we’re wearing masks, whether we’re singing, and even how heavily “clustered” the immune people are. The complexity in all those factors makes it hard to be exact about what the threshold is.

As we get closer to the herd immunity threshold, we will likely start to see case numbers dropping. 📉 Maybe dropping really fast! This is called exponential decay. That is something to look forward to because it means less sick people, obviously–but it also means we’re headed for herd immunity. We might already be seeing this in some parts of the world with the highest vaccine coverage.

We can easily imagine a situation where we have reached the herd immunity *threshold* but still do not have herd immunity. Let’s think through some examples:

1️⃣ Imagine a town of 10,000 people, called Faketown. Imagine a future day late this summer when 8,000 out of 10,000 people in Faketown are fully vaccinated. The other 2,000 are not. 1 of those people is allergic to PEG, which is an ingredient in the vaccines, so she can’t be vaccinated. Her name is Prisha. The other 1999 people just don’t want to be vaccinated. Let’s also pretend that as of August, there are zero cases of COVID-19 in Faketown, just because they’re lucky.

Alex is a 14 year old. He and his sister and their two parents are some of the unvaccinated people who live in Faketown. They visit family in Bolivia for the summer. Bolivia does not have 8 out of 10 of their people vaccinated–in fact they barely have 1 in 10. When Alex and his family come back from Bolivia, Alex brings back COVID-19 too. Alex gives it to two other kids in his school who aren’t vaccinated. Not being vaccinated tends to be a family decision, so those other kids both go home and give it to their unvaccinated parents and siblings. 👨‍👩‍👧‍👧 The siblings take it to their own classrooms. The parents take it to their workplaces. More people are exposed.

Alex’s dad gives it to his grandfather, who WAS vaccinated but is in chemotherapy, so his immune system isn’t working very well. He has a “breakthrough” case, where a vaccinated person gets COVID-19 anyway. This is rare, but it happens with all vaccines.

Alex’s mom takes it to work, where she unknowingly exposes Prisha, her officemate.

Prisha is NOT protected from being exposed to COVID-19. An outbreak can still happen even though the herd immunity threshold has been reached in Faketown. This is a situation where we have reached the herd immunity *threshold* but we have not yet reached herd immunity. One case turns into many more, even though 8 out of 10 people in Faketown are vaccinated.

2️⃣ Now, here’s an example of herd immunity and herd immunity threshold together: imagine another town. This one is called Modville. Modville has 10,000 residents. 9,500 of them have had their measles vaccine, or are old enough to have had wild measles as a child–or both.

Jaime is 2. He has leukemia and has no immune system. He can’t be vaccinated safely. The other 499 unvaccinated people are children under the age of 1. They aren’t old enough to have had a measles vaccine.

Now, baby Lila is not vaccinated because she’s only 5 months old. She and her family travel to Kenya, where measles is common, and a few days after they return, Lila comes down with measles. Lila’s 2 year old brother Idris is vaccinated, and so are her parents, so none of them get sick. Lila and Idris go to their aunt’s home for childcare, but her aunt and her whole family are all vaccinated. Lila’s parents take her to the doctor’s office, where every adult she encounters is immune to measles. Lila’s measles case does not turn into any more measles cases–even though measles is the most infectious disease there is.

Jaime is not going to get measles unless Jaime is very, very unlucky. He is protected by herd immunity, even though he is not protected by a vaccine.

If Lila were to spend time with other babies under age 1, she could give them measles, and it could turn into a small outbreak. But it would quickly fizzle out because just about everyone who interacts with the babies in Modville has had their measles vaccine.

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