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Last updated February 18, 2021. 6 minute read

Why is the COVID-19 vaccine given in two doses?

The COVID-19 vaccines currently available in the United States require two doses each. One dose of each is about 50% effective at preventing infection. Research has shown that a second dose of each boosts your immune system’s ability to respond to the virus, making it around 95% effective.

Written by Yael Cooperman, MD
Reviewed by Steve Silvestro, MD

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 coronavirus disease 2019 (COVID-19) vaccines, we’re exposed to tiny portions of the virus’s genetic material, which lets our immune system learn to identify the virus without actually causing disease. 


  • The COVID-19 vaccines currently available require two doses each. Because of how our immune systems work, some vaccines require just one dose while others require two or more doses. 
  • For the COVID-19 vaccines (made by Pfizer-BioNTech and Moderna), the first dose primes your immune system to recognize the virus and the second dose strengthens your immune memory so it can effectively fight off infection.
  • One dose of each vaccine is about 50% effective at preventing infection. Research has shown that a second dose of each boosts your immune system’s ability to respond to the virus, making it around 95% effective.

Why do I need two doses of the COVID-19 vaccine?

As COVID-19 vaccines roll out, you may be wondering why some vaccines require two doses, while others are effective after just one dose. 

The first two vaccines to receive Emergency Use Authorization (EUA) from the U.S. Food and Drug Administration (FDA) are the Moderna vaccine and the Pfizer-BioNTech vaccine. Each of these vaccines requires two doses to reach their reported levels of effectiveness. 

The vaccines stimulate a slightly different immune response, and while one dose gives you significant protection from COVID-19, researchers found that a second dose is most effective at eradicating COVID-19 infections. That’s because each exposure to the vaccine strengthens your body’s ability to mount a response to the virus. 

When our immune system is exposed to something foreign (like a virus, or the vaccine in this case), 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 about four weeks. Why? It isn’t a failing of our immune system, but 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, 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).  

When should you get your first and second doses?

The Moderna and Pfizer-BioNTech vaccine trials only followed people who got doses at the recommended gaps of 28 and 21 days apart, respectively. Researchers did not continue monitoring people with different gaps. For that reason, these are the recommendations for the time being. 

This doesn’t mean that other time gaps aren’t favorable. It just means that we don’t have sufficient information to make recommendations about alternatives. But the more people receive these vaccinations, the more information researchers glean about how effective the vaccines are, and how far we can stretch the limits. 

In fact, an analysis by the Centers for Disease Control and Prevention (CDC) found that a 42 day (six-week) gap is also acceptable. However, deviations from this schedule aren’t necessarily ineffective, it’s just a reflection of the information we have (CDC, 2021). 

Is there an ideal time amount between doses?

It depends. In some regards, longer is better. It gives a chance for the immune system to develop an ideal response before being exposed again, maximizing long-term response (Siegrist, 2016). 

Here’s the problem. After getting the first dose of the COVID-19 vaccine, you’ll achieve partial protection from the virus. But if you then catch COVID-19, you’re the host for a theoretically more infectious version of a virus—which is changing constantly—that the vaccine may be less effective. 

That’s why we need optimal time between shots. When it comes to multi-dose vaccines, the interval should be at least three weeks between the first and second dose to give optimal results. It’s also why (for now at least) it’s critical for people to continue wearing masks, remain socially distanced, and stay home whenever possible—even after being vaccinated.

Another important factor in understanding the immune response is the age you get vaccinated. If you’ve ever been pregnant, for example, you may have received what’s called a pertussis vaccine. This vaccine protects against a condition called whooping cough, which isn’t typically dangerous if you catch it as an adult. If you catch it as a baby, however, it can be deadly. 

Unfortunately, some vaccines can’t stimulate a sufficient immune response in infants because a baby’s immune system is too immature to respond. Luckily, antibodies can cross the placenta and reach the baby before it’s born. For that reason, pregnant people receive vaccines that give the baby protection from whooping cough for the first few months after it’s born. 

This all illustrates why vaccines are so crucial to halting the spread of the coronavirus. The faster people get vaccinated, the faster things can get back to normal—or as normal as things can be. As vaccines are distributed, keep track of the eligibility of you and your loved ones so that we can all protect each other. 

How effective are the COVID-19 vaccines currently available?

Thanks to modern technology and massive studies done on large groups of people, there’s a lot we know about the COVID-19 vaccines. Both are considered safe and very effective at preventing infection and severe COVID-19 symptoms.

But there are some things we don’t know, like how long protection from the coronavirus actually lasts. For example, existing data shows 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). We see the same thing with the flu shot as well. An annual flu shot grants about 50% protection from the influenza virus, but its true effectiveness varies between men, women, and across different age groups. 

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 less 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 (CDC, 2018). 

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 the merrier.