COVID-19: What Will It Take to Finally Achieve Herd Immunity?

Caesar Djavaherian, MD, MS, FACEP
February 18, 2021
4 minutes

How Many People Need to be Vaccinated Before We Reach Herd Immunity?

This critical question has no easy answer. First, let’s define herd immunity. It’s the point at which enough people are vaccinated, or immune, to a virus or disease that those who cannot be vaccinated – the medically fragile, young babies, children, and others – are protected because the disease can’t easily spread.

The CDC and other health agencies are reluctant to say exactly how much of the U.S. population needs to be vaccinated to achieve herd immunity. This percentage has never really been an exact number; it varies by disease. For measles, for example, 95% vaccination is required and for polio 80% is required to achieve herd immunity, according to the World Health Organization.

At this time, the best approach is to continue large-scale roll-outs of the vaccination and monitor the subsequent drops in cases and deaths. Some early models indicate that a 40% vaccination rate may cut the rate of new infections four-fold, but with only about 11% of the population getting one dose and less than half that getting a second dose, so far, it’s still too early to tell.

As of now, nearly 53 million vaccines have been administered, according to the CDC’s COVID tracker, and authorities are working to increase shipments to states and further speed along with the rollout. The expectation is that there will be enough vaccines available to immunize 300 million Americans by the end of summer, which covers about 90% of the U.S. population.

I Got the COVID-19 Vaccine. How Long Until I’m Immune?

First, there are two coronavirus vaccines that received emergency approval in the U.S., one from Pfizer and the other Moderna. The overall immune response for both has been about the same. Both claim a roughly 95% rate of effectiveness after two doses. You should expect to reach your full protection about two weeks after your second dose for both vaccines.

A common follow-up question I hear is: Isn’t one good enough? The answer is an emphatic, “no.” Even though one shot is better than none, you must get your second dose.

The Pfizer vaccine is believed to offer about 52% effectiveness after the first dose and 95% after the second dose. The Moderna vaccine sees a roughly 92% effectiveness two weeks after the first dose, but it’s still recommended to get your second dose to achieve that 95% level. The second dose for the Pfizer vaccine is recommended 21 days after the first, and the recommendation for the second Moderna shot is 28 days after the first dose.

Do Antibodies Make You Immune to COVID-19?

I hear this question often, and it’s important for patients to understand the antibody-immunity connection. When a healthy immune system encounters a spike protein from a virus, it creates antibodies to fight the virus, regardless of whether the spike protein is introduced by the mRNA vaccine, a different type of immunization, or transmitted through respiratory droplets.

How long antibodies stay in place and how long you maintain immunity varies for every disease. We’ve learned so much about COVID-19 in a short time, but we still need to gather more data about long-term immunity. For the time being, however, we know the body’s immune response to the vaccine is very strong. We’re confident future data will confirm our thinking that those who have taken the vaccine will enjoy long-term immunity.

I've Already Had COVID-19. Am I Immune?

If you’ve had COVID-19 and recovered, your body has produced antibodies to the virus and summoned the necessary response to make you immune. However, because we are not certain about how long natural immunity lasts and there is a very slight chance for reinfection, we are still encouraging you to get the vaccine.

New strains of COVID-19 have started to surface in the United Kingdom, South Africa, and other countries around the world. This occurs with many viruses, and while we still need more data to confirm it, there is a belief that the vaccines could provide added protection against new strains. If you’ve had COVID-19 recently, you should not get the vaccine until at least 90 days after your positive test.

While there’s still more to learn about the COVID-19 vaccines and data to be gathered on our immune response, the early signs are that researchers have accomplished quite the feat in developing a vaccine this effective in less than a year – a process that usually takes many years.

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Caesar Djavaherian, MD, MS, FACEP

Caesar Djavaherian, MD FACEP, is the Chief Clinical Innovation Officer at Carbon Health. 


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