Why do you need multiple doses of the COVID-19 vaccine?

Hundreds of years ago, when smallpox ran rampant throughout Europe, people noticed that milkmaids (those ladies who sat on little stools and milked cows) seemed impervious to the awful affliction. 

One scientist, Edward Jenner, theorized that it was because these women sometimes came down with a similar, but much milder illness, they caught from the cows known aptly as cowpox (Morgan, 2013). To test the theory, he did what anyone in an age devoid of medical ethics, regulations, or accountability would do: scraped some pus from a boil on the arm of a milkmaid infected with cowpox and exposed a young boy to the pus. Then he exposed the child to smallpox, and wouldn’t you know it? The kid was immune. 

And thus was born the very first vaccine. This primitive prophylactic managed to eradicate a disease that ravaged the human population, killing upwards of 500 million people over the last 100 years of its existence. Finally, in 1980, smallpox was declared a thing of the past by the World Health Organization (CDC, 2016). 

Luckily, science has come a long way when it comes to vaccines. Thanks to modern technology and a lot of brilliant scientists, the old method of exposing people to large quantities of infectious viruses is gone. Instead, as is the case with the COVID vaccines, we’re exposed to tiny portions of the outside of the virus, which lets our immune system learn to identify the virus without actually causing disease. 

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The reason that we need multiple doses of the COVID vaccine for optimal protection is a function of how our immune systems work. The first time you’re exposed to some component of a virus (or the virus itself), a specific type of immune response is triggered. And while that response is often a strong one, it can wane with time if you aren’t exposed again. Our immune systems have limited resources and benefit from focusing on protecting us against things that we see frequently rather than bolstering immunity against things our system has only seen once.

When our immune system is exposed to something foreign (like a virus, or a component of the virus in the case of the vaccine), our immune system produces antibodies. Antibodies work like little magnets and stick to infected cells. These antibodies then recruit other players from our immune system, which come in and destroy the cells infected with the virus. 

Unfortunately, this first response is short-lived and antibodies levels drop after several weeks. Why? It isn’t a failing of our immune system, but rather a mechanism that ensures that our bodies don’t waste time and energy focused on something that isn’t around anymore. 

Upon second exposure to the same virus or vaccine, our immune system launches a stronger response. This second response peaks much faster, reaching impressive levels of protection in just seven days. More importantly, it creates an immune memory that lasts for months or even years (Siegrist, 2016).  

The longer the pandemic lasts, the more data researchers are able to collect about the virus, the disease it causes, and the effectiveness of the vaccines that have been widely used. Researchers looked at nursing home residents in the US and found that protection against infection with the virus began to fall about six months after the second dose from around 75% to around 53% (Nanduri, 2021). But protection against serious infection and protection against the need for hospitalization actually remained high (around 90–95%) (Rosenberg, 2021). 

Still, booster doses are important for limiting the spread of the virus. Research showed that a third dose of the vaccine after six months reduced the rate of serious infection and the need for hospitalization significantly (Barda, 2021). This pattern of immunity following vaccination is common when it comes to viral infections. The flu, for example, is a virus that changes frequently and these changes allow the virus to escape our immune system. That’s why we need to get vaccinated every year for the flu with a new version of the vaccine. 

Thanks to modern technology and massive studies performed on large groups of people, there’s a lot we know about the COVID vaccines. Both are considered safe and very effective at preventing infection and severe COVID symptoms.

But there are some things we don’t know, like how long protection from the coronavirus actually lasts. For example, early data showed that the first dose of the Pfizer vaccine provides about 50% protection, while the second dose boosts it into the 95% effectiveness range (Pfizer, 2020). Additional research shows that even the first dose might give 75% protection against infection within a few weeks of getting it (Amrit, 2021). That sounds pretty good, but widely accepted thinking about vaccines suggests that a second dose, in addition to giving better protection, would also give longer-lasting immunity.

Another bonus to getting vaccinated is the vaccine doesn’t just protect the person who gets it. It can protect entire communities, including those who can’t or won’t get a vaccine. The fewer people able to get infected, the less likely the virus is to find a viable host. Eventually, the bug disappears altogether—a phenomenon called herd immunity. 

Herd immunity protects both people who can’t get vaccinated and the portion of the population for whom the vaccine isn’t quite as effective. Researchers estimate that in order to reach a state of herd immunity from the coronavirus, we’ll need somewhere around 70% of people to be protected (whether through infection or vaccination) (Nayer, 2020). But, of course, the more people that can get vaccinated, the better.

This all illustrates why vaccines are so crucial to halting the spread of the coronavirus and why it’s so important to vaccinate as many people as possible, as quickly as possible. As different variants arise, guidelines are likely to change. Make sure to keep track of the most up-to-date guidelines to be sure to protect yourself and others.

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Current version

November 08, 2021

Written by

Yael Cooperman, MD

Fact checked by

Steve Silvestro, MD

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About the medical reviewer

Steve Silvestro, MD, Ro

Dr. Steve Silvestro is a board-certified pediatrician and Associate Director, Clinical Content & Education at Ro.