How many people have to get vaccinated to reach herd immunity? Will we ever get there?


A: About 70-90% of the population will need to get the COVID-19 vaccine to stop transmission.

If vaccine acceptance is low, it could take until late 2022 to reach herd immunity. If vaccine acceptance is high, we could be well on our way by this summer. The good news: people are reporting less hesitancy and higher willingness to get vaccinated.

🧮Let’s start with a definition and some simple math. What is herd immunity? It’s the protection conferred on a community when enough individuals are immune to a disease that the disease can’t spread — it can’t find enough “fuel” (in the form of susceptible people) and fizzles out.

🦠The herd immunity “threshold” (the proportion of people who need to be immune from a disease before herd immunity kicks in) can be estimated using R0 (pronounced “R-naught”), aka the basic reproduction number of an infectious pathogen. R0 tells you how many additional people each infected person is likely to infect. If R0 = 4, then 1 infected person will infect 4 more on average. (Note that R0 doesn’t take into account mitigation measures like masking or distancing.)

🔢 Once you know R0, you can use this formula to calculate the herd immunity threshold: 1 – (1/R0). The R0 of SARS-CoV-2 is somewhere between 2.5 and 6, which puts the herd immunity threshold at 60-83% (see links below for a more detailed walk through the math, and an article about how our understanding of the R0 of SARS-CoV-2 has changed over time).

💯Ok, so far so good. But then, why the 70-90% estimate for vaccine coverage before we hit herd immunity? That number has to be higher than the herd immunity threshold, because vaccines don’t provide 100% protection. In the case of the two authorized mRNA vaccines from Pfizer and Moderna, the vaccines look to provide about 90-95% protection, which is amazing. But we still need more people to be vaccinated than the herd immunity threshold to account for effectiveness of less than 100%.

(See link below for a story on shifting estimates of vaccine coverage needed to reach herd immunity, including comments by Dr. Tony Fauci on why his estimates have increased.)

😷What about all the people who have already had COVID-19? Can’t we count them as having acquired immunity? Unfortunately, we just don’t know enough yet about the level and duration of immunity conferred by disease.

For now, people who have had COVID-19 are still encouraged to get vaccinated when eligible.

Back to the original question: When will we get to herd immunity?? This depends on two things in addition to the vaccine effectiveness factor described above:

1️⃣ 💉 Vaccine supply: How fast can vaccine doses get manufactured, allocated, distributed, and administered? According to the CDC (see link to updated data below), 11.4 million doses have been distributed to state and local health departments (who are responsible for vaccine delivery), and 2.1 million doses have been administered. 11.4 million doses is about 2.4% of the doses we need to cover 70% of the US population with two doses each. Hopefully the pace of vaccine manufacturing and distribution will pick up over the next several weeks.

2️⃣ 👍Vaccine demand: To reach herd immunity, people need to be willing to get the vaccine. On that front, the news is looking good: In general, surveys are reporting higher levels of willingness to get the vaccine than were reported over the summer and fall (links below for more on this trend). Vaccine demand will be a major driver of how long it takes to get to herd immunity. One estimate of how long it will take to reach herd immunity compared different vaccine acceptance scenarios (link below): At 39% acceptance, the US reaches herd immunity in December 2022. At 61% acceptance, we reach herd immunity in July 2021 (conditional on supply, plus a lot of other assumptions.)

Until it’s your turn to get vaccinated, #staySMART! And if you’re involved in vaccine distribution, delivery, and promotion in your community, tell us about it below!


CDC vaccine tracker

STAT News on herd immunity math and timing estimates

MarketWatch on how many people have to get vaccinated + declining hesitancy

NYT on changes in herd immunity threshold + Fauci comments + declining hesitancy

Link to original FB post