Got Your First Dose of the COVID Vaccine? Yes, You Still Need the Second Shot

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


Medically reviewed in March 2021

Updated on February 19, 2021

Millions of people have already lined up to get either the Pfizer or Moderna COVID-19 vaccines. Both require two doses, according to the manufacturers and the Centers for Disease Control and Prevention (CDC). The second dose of the Pfizer vaccine is given 21 days after the first shot, and the second dose of the Moderna vaccine is given 28 days after the first.

As of February 19, more than 41 million Americans have received their first doses and more than 16 million have gotten their second, the CDC reports.

But many people who have received their first dose are questioning whether they need—or want—that second shot.

Reports of serious side effects, vaccine shortages and misconceptions about the effectiveness of a single dose are some of the reasons why people are thinking twice about rolling up their sleeve again.

Meanwhile, new research into the Pfizer vaccine published on February 18, 2021 in The Lancet suggests that getting only the first shot may be nearly as effective as getting the prescribed two doses.

Researchers at Sheba Medical Centre in Israel looked at data of 9,109 healthcare workers at the hospital, both vaccinated and unvaccinated. The study authors found that one shot of the Pfizer vaccine was 85 percent effective in preventing symptomatic cases of COVID-19 within 15 to 28 days after injection. Overall, getting one shot was 75 percent effective for both symptomatic and asymptomatic cases. These numbers may seem to compare favorably to the 95 percent efficacy rate for the two-dose regimen reported in Pfizer’s original clinical trial.

But does this mean you can get just one dose and skip the second? Not so fast.

The CDC still recommends that those receiving the Pfizer vaccine get both doses, ideally 21 days apart and up to 42 days apart if a delay is unavoidable.

What’s more, the Israeli authors noted that the people in the study were generally young and healthy and therefore not representative of the broad swaths of the population getting the vaccine. And crucially, while the short-term effectiveness of one dose of the Pfizer shot may be encouraging, the study does not say how long that protection will last.

The upshot: It’s still important to get both doses of the vaccine. Here’s why.

That second shot is key for building immunity
The second dose of the available mRNA COVID-19 vaccines is important. While the first dose prepares or primes the immune system, the second dose is needed to develop long-term protection.

Pfizer’s Phase 3 trials showed that one dose of its coronavirus vaccine was 52 percent effective after 21 days. Efficacy, however, jumped to 95 percent 7 days after receiving a second dose of the vaccine.

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

While 80 percent may seem relatively high, it’s important to understand that these results are limited by a small sample size and short follow-up period. Although there seems to be some protection after one shot, it’s still unclear how effective one dose is beyond 28 days.

Side effects are normal—and temporary
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.

Though some people have reported side effects after the first dose, many more have reported these issues after their second dose. Though unpleasant, these mild to moderate side effects aren’t a reason to worry or skip dose two. They’re not only normal—they are a reassuring sign that the vaccine is working. Moreover, these side effects will typically disappear within a few days.

For millions of people, the symptoms of COVID-19 are much worse, as is the risk of serious illness. As of February 19, the disease has claimed more than 2.4 million lives around the world. Getting fully vaccinated is not only critical to protect yourself, it’s also crucial to protect your loved ones and your community.

Why some side effects are normal
We all know that vaccines are developed to help the immune system ward off infections. But understanding how and why you develop side effects could help ease your concerns and reinforce the importance of going back for your second dose.

What happens after dose 1: The vaccine triggers your innate immune response, the body’s first line of defense against germs. These fast-acting immune cells release an army of cytokines, or small proteins, that affect interactions and communication between cells.

This initial response can result in fever, swelling or other temporary side effects.

What happens after dose 2: Some time after the first dose, the innate immune system gives way to the adaptive immune system, which involves T cells and B cells that remember previously identified germs and make antibodies to attack them.

To be clear, the innate and the adaptive system are continually working together to help fight infection. But the body’s reaction to dose 2 of a vaccine may be amplified because of this adaptive immune response. In other words, when it’s time for dose 2, not only is the innate response triggered but adaptive immune cells are ready to jump into action as well for a more robust immune response.

On the flip side, experiencing few or no side effects doesn’t mean the vaccine isn’t working. Everyone reacts a little differently to vaccines. Some people experience few or no side effects, which may just mean their immune system is building up its defenses against the disease.

Skipping a second dose has consequences
In January, the U.S. Food and Drug Administration (FDA) warned against skipping the second dose of either the Moderna or Pfzier vaccine, noting that it would be "premature and not rooted solidly in the available evidence."

The FDA added that the Phase 3 trials of these vaccines studied two doses—not one. Getting only one of two doses would be worrisome because it’s unclear how much protection you’d get from that one shot and how long it would last.

In addition, people who skip their second dose may not be fully protected but assume that they are immune. This could lead them to engage in risky behavior which could not only put them at risk but also hinder efforts to stop the spread of COVID-19.

Mutations are even more reason to be fully protected
It’s normal for viruses to mutate, which is why it’s critical that everyone eligible for a COVID vaccine get fully immunized as quickly as possible, according to Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases (NIAID).

Some scientists who study viruses speculate that if people skip the second shot of two-dose coronavirus vaccines, it could allow the virus keep mutating. This could render existing vaccines less effective against it.

Being fully vaccinated can help prevent this from happening, according to Dr. Fauci.

“The best way to prevent the evolution of mutations is to suppress the replication of the virus in the community, which means that we need to vaccinate as many people as quickly as we possibly can and as efficiently as we possibly can,” he said during a February 2 White House COVID-19 response team press conference. "Vaccinating people, you diminish its ability to mutate.”

Meanwhile, vaccine developers, including Pfizer and Moderna, are already working to update their COVID vaccines with added protection against the new variants.

How you can prepare for your second shot
Being aware of possible side effects can help you prepare for them. If you develop pain or discomfort after receiving your vaccine, ask your doctor if it’s safe for you to take an over-the-counter pain reducer, such as acetaminophen. 

Keep in mind, some research suggests it’s a good idea to avoid certain medications, including ibuprofen, right before or after being immunized because they could interfere with your immune response to the vaccine.

Be sure to continue taking any medications or low-dose aspirin as directed or prescribed by your healthcare provider (HCP).

Other ways to ease uncomfortable side effects:

  • Apply a clean, cool, compress over the injection site.
  • Move or use the arm that received the shot to increase blood flow to the area and to help distribute the vaccine through your body.  
  • Drink plenty of fluids.
  • Dress in light, comfortable clothing.

It’s also wise to stock up on groceries and other essentials before receiving your vaccine so you’ll have them on hand if you’re not feeling well.

When to call your doctor
Side effects from a COVID vaccine should disappear within a few days. Be sure to call your healthcare provider if you develop increased redness or tenderness at the injection site after 24 hours, or your side effects persist for more than three days.

Severe allergic reactions to the Pfizer or Moderna COVID vaccines are rare. But after receiving one of these vaccines, the CDC advises that most people be monitored for about 15 minutes to make sure they don’t experience a serious allergic reaction.

Those with a history of a life-threatening reaction, called anaphylaxis, should be monitored for 30 minutes after being vaccinated. Warning signs of anaphylaxis include:

  • Swelling
  • Trouble breathing
  • Skin rash or hives
  • Tightness in the throat or a hoarse voice
  • Nausea, vomiting or belly pain
  • Diarrhea
  • Dizziness or fainting
  • Low blood pressure

If you think you may be having a severe allergic reaction following a COVID-19 vaccine, you should call 911 right away and seek immediate medical attention.

Anyone who experiences a life-threatening allergic reaction to their first dose of a two-dose COVID vaccine should not receive their second dose.

Article sources open article sources

Sharon Amit, Gili Regev-Yochay, et al. “Early rate reductions of SARS-CoV-2 infection and COVID-19 in BNT162b2 vaccine recipients.” The Lancet. February 18, 2021.
Centers for Disease Control and Prevention. “COVID-19 Vaccinations in the United States.” Feb 3, 2021.
U.S. Food and Drug Administration. “Moderna COVID-19 Vaccine EUA Fact Sheet for Recipients and Caregivers.” December 2020.
U.S. Food and Drug Administration. “Pfizer-BioNTech COVID-19 Vaccine.” Feb 3, 2021.
Centers for Disease Control and Prevention. “What to Expect after Getting a COVID-19 Vaccine.” Jan 11, 2021.
Johns Hopkins University and Medicine. Coronavirus Resource Center. Feb 4, 2021.
U.S. Food and Drug Administration. “FDA Statement on Following the Authorized Dosing Schedules for COVID-19 Vaccines.” Jan 4, 2021.
Mahase Elisabeth. “Covid-19: Pfizer vaccine efficacy was 52% after first dose and 95% after second dose, paper shows.” BMJ 2020.
Clem AS. “Fundamentals of vaccine immunology.” J Glob Infect Dis. 2011;3(1):73-78.
U.S. National Library of Medicine. “How Vaccines Work.”
U.S. Department of Health and Human Services. “Vaccines Protect You.” Mar 2020.
Centers for Disease Control and Prevention. “Understanding How Vaccines Work.” Jul 2020.
U.S. Food and Drug Administration. “FDA Statement on Following the Authorized Dosing Schedules for COVID-19 Vaccines.” Jan 4, 2021.
U.S. National Institutes of Health. “Dr. Anthony Fauci on COVID-19 Vaccines.” Aug 2020.
UD Davis Health. “How the COVID-19 vaccine works, potential side effects and more.” Jan 26, 2021.
Nature. “How to redesign COVID vaccines so they protect against variants.” Jan 29, 2021.
Centers for Disease Control and Prevention. “What to Expect after Getting a COVID-19 Vaccine.” Jan 11, 2021.
Saleh E, Moody MA, Walter EB. “Effect of antipyretic analgesics on immune responses to vaccination.” Hum Vaccin Immunother. 2016;12(9):2391-2402.
Centers for Disease Control and Prevention. “Vaccine Safety Monitoring and Reporting in Your Facility.” Dec 23, 2020.
American College of Allergy, Asthma & Immunology. “Anaphylaxis.” Jan 29, 2018.
Centers for Disease Control and Prevention. “COVID-19 Vaccines and Allergic Reactions.” Jan 22, 2021.

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