Don’t Skip Your Second COVID Shot

Here’s why it’s important to get your second dose of the Pfizer or Moderna vaccine.

woman getting vaccinated

Updated on April 26, 2021.

A little more than a year ago, there were fewer than 100 confirmed cases of COVID-19 in the United States. Fast forward to April 2021, more than 31 million Americans have been diagnosed with the disease, including more than 568,000 who lost their lives as a result.

The world is much better equipped to manage the coronavirus now than it was was the pandemic began.  This arsenal includes three vaccines, each of which has been shown to be safe and highly effective in preventing severe disease and death.

Two of these vaccines—Pfizer and Moderna—are mRNA vaccines—a newer type of vaccine technology that uses mRNA to deliver instructions on how to make one protein from the coronavirus, which triggers an immune response. In clinical trials, the Pfizer vaccine was shown to be 95 percent effective against COVID-19 while Moderna's had an efficacy rate of 94.1 percent.

Under real-world conditions (outside of tightly controlled trials), the mRNA COVID-19 vaccines developed by Pfizer and Moderna are 90 percent effective in preventing the disease, according to data released on March 29 by the Centers for Disease Control and Prevention (CDC).

But reaching this high level of protection requires not just one—but two doses of these vaccines.

This second dose isn’t just for good measure or “extra protection.” If you’re considering skipping that second shot because you think it’s unnecessary, inconvenient or you’re worried about stronger side effects, think again.

Here are 5 reasons why you need both doses of these COVID-19 vaccines:

1. Clinical trial results are based on two doses—not one. Do you like the sound of high efficacy rates like 94.1 and 95 percent? Then get your second shot. These rates are based on trial results which showed these vaccines are most effective after two doses spaced a few weeks apart. Two shots are a lot more effective than one.

A study of more than 43,000 people found that one dose of its coronavirus vaccine was only 52 percent effective after 21 days. Efficacy, however, jumped to 95 percent 7 days after receiving a second dose of the vaccine. Get your second shot.

Similarly, one dose of Moderna’s vaccine was shown to be only 80 percent effective after 28 days. But another two weeks after a second dose, its efficacy hit 94.1 percent. Again, get your second shot.

You may think 80 percent—or even 50 percent—seems “good enough” but it’s not safe to assume this. Although there seems to be some protection after one shot, it’s still unclear how effective one dose is beyond 28 days.

2. That second dose is necessary to develop long-term protection. It's not unusual for vaccines to require two or even three doses to be most effective.

For example, kids are scheduled to receive two or more doses of the chickenpox vaccine, the MMR vaccine, the Hepatitis A and B vaccines among others.

Why is this important? The first dose of a vaccine primes the immune system while the second dose triggers a more potent immune response. If kids don’t get their second doses, they are at greater risk for catching these diseases. Similarly, you need both doses of the Pfizer or Moderna vaccine to be protected from COVID-19.

3. Vaccine side effects are normal—and temporary. Side effects can be temporarily uncomfortable, but they aren’t a reason to skip dose two. They’re not only normal—they are an outward sign that the vaccine is working. Moreover, these side effects will typically disappear within a few days.

Fever, fatigue, headache, muscle and joint pain, chills, nausea and vomiting are among the most commonly reported side effects of the Moderna and Pfizer COVID-19 vaccines. Some people may feel like they have the flu for a short time. Some people experience no side effects at all.

4. Skipping a second dose can cause harm. If you skip your second shot of the Pfizer or Moderna vaccine, you may not be fully protected but assume that you are immune. You may develop a false sense of security, let your guard down and take risks that could not only put you at risk for infection, but fail to stop the spread of the coronavirus, which is the only way we know to end the pandemic.

5. Just getting one shot could enable the coronavirus to keep mutating. Some scientists who study viruses speculate that if people skip the second shot of two-dose coronavirus vaccines virus may continue to mutate. This could render existing vaccines less effective against it. If you’ve already gotten one dose of the Pfizer or Moderna COVID-19 vaccine, getting your second shot as scheduled will help prevent this from happening.

Article sources open article sources

Centers for Disease Control and Prevention. “COVID Data Tracker.” March 2, 2021.
Centers for Disease Control and Prevention. “Different COVID-19 Vaccines.” Jan 15, 2021.
Centers for Disease Control and Prevention. “Media Statement from CDC Director Rochelle P. Walensky, MD, MPH, on Signing the Advisory Committee on Immunization Practices’ Recommendation to Use Janssen’s COVID-19 Vaccine in People 18 and Older.” Feb 28, 2021.
Lindsey R. Baden, M.D., Hana M. El Sahly, M.D., Brandon Essink, M.D. et al. Efficacy and Safety of the mRNA-1273 SARS-CoV-2 Vaccine. N Engl J Med 2021; 384:403-416.
Moderna. “Moderna Announces Primary Efficacy Analysis in Phase 3 COVE Study for Its COVID-19 Vaccine Candidate and Filing Today with U.S. FDA for Emergency Use Authorization. Nov 30, 2020.
Mahase Elisabeth. Covid-19: Pfizer vaccine efficacy was 52% after first dose and 95% after second dose, paper shows BMJ 2020; 371. Dec 11, 2020.
U.S. Food and Drug Administration. “Vaccines and Related Biological Products Advisory Committee Meeting: December 17, 2020 FDA Briefing Document Moderna COVID-19 Vaccine.” Dec 17, 2020. “Vaccine FAQ.” Mar 1, 2021.
American Academy of Family Physicians. Recommended Adult Immunization Schedule—United States.
Clem AS. Fundamentals of vaccine immunology. J Glob Infect Dis. 2011 Jan;3(1):73-8.
Centers for Disease Control and Prevention. “What to Expect after Getting a COVID-19 Vaccine.” Feb. 23, 2021.
Livingston EH. Necessity of 2 Doses of the Pfizer and Moderna COVID-19 Vaccines. JAMA. 2021;325(9):898.
Nature. “How to redesign COVID vaccines so they protect against variants.” Jan 29, 2021.

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