Early COVID-19 Vaccines Will Require Two Shots (2024)

For people receiving the first wave of COVID-19 vaccinations, the process will not be over with one shot.

Both the Pfizer/BioNTech and Moderna vaccines, the two vaccine candidates currently seeking Food and Drug Administration (FDA) authorization, require two doses. As is true with many multi-dose immunizations, the first dose of these vaccines will help prime the immune system to create a response against SARS-CoV-2, the virus that causes COVID-19. Subsequent shots will help boost that immune response.

On Thursday, a panel of experts recommended that the FDA approve a vaccine developed by Pfizer/BioNTech. It was found to be 52% effective 10 days after the first dose, according to the FDA trial data. This efficacy increases to 95% with the second dose. These efficacy results are “similar to some of the very best vaccines we have,” Richard Kennedy, PhD, co-director of Mayo Clinic’s Vaccine Research Group, tells Verywell.

Despite the significant efficacy of the first dose, experts say the second one is to ensure successful and long-term protection.

How the Doses Differ

The Pfizer and Moderna vaccines are genetic vaccines. They are composed of tiny capsules containing messenger RNA (mRNA)—the code that directs protein production in cells. This mRNA enters certain cells and instructs them to create proteins like the spike protein on the exterior of the SARS-CoV-2 virus.

Helper T-cells will recognize the spike proteins as belonging to a pathogen and raise the alarm to the immune system. If other immune cells, called B-cells, interact with the protein from vaccinated cells and are activated by the T-cells, they will begin to produce antibodies. These antibodies can recognize and latch onto the coronavirus spikes to keep them from harming healthy cells.

The first dose of a vaccine jump-starts this immune response by spurring the production of antibodies. This is the body’s first opportunity to recognize and defend against the virus. B-cells and memory T-cells can then recognize the virus for weeks, months, and possibly years.

The second dose helps the body to create even more antibodies, thereby strengthening the immune response. According to Kennedy, there is no difference between the doses; you just need two of them.

For some people, the immune response after the first dose isn’t great enough to provide strong or lasting protection. Others’ bodies may not respond at all. “We give two doses to make sure we give everybody two chances for their body to create a response,” Kennedy says.

Timing the Doses

After receiving a dose of the vaccine, it takes a week and a half to two weeks before the body has created enough T-cells and B-cells to create an immune response. After three to four weeks, the immune response is at its peak, Kennedy says. That’s when a vaccine recipient gets a booster shot—or their second dose.

The timing of the dosing schedule for a certain vaccine depends on factors like its type (genetic, live virus, or other) and the amount of viral protein it contains. Clinical trials show the Pfizer vaccine is most effective if the second dose is issued 21 days after the first, and the Moderna vaccine was studied with the two doses administered 28 days apart.

To determine the ideal period between doses, vaccine manufacturers typically refer to results from as early as the pre-clinical animal trials, according to Kennedy. While timing recommendations may lead to the greatest efficacy, they may not need to be followed precisely.

“If they can’t get it on day 21 but they can get it on day 40, they should go ahead and get it on day 40," Kennedy says. "Twenty-one isn’t a magic number. In the vaccine manufacturer’s experience, that’s the best time. And there’s probably a reasonable range—a window of opportunity—before and after.”

Is the Second Dose Necessary?

The FDA trial data published this week indicates that one dose of the vaccine is more than 50% effective at providing protection against COVID-19. This rate, however, only accounts for protection 10 days after the first dose is received. Kennedy says that some people may not create an immune response at all, or it may not be strong for a long time.

“You have no idea if you’ll be the person who does or does not respond properly to the first dose unless you then go and get antibody tested,” Kennedy says. “At a public health and population level, it’s a lot easier and more straightforward for everyone to get the two doses.”

Even if a person’s immune system does create a response to the first dose, scientists don’t yet know for sure how many antibodies are necessary for a proper, prolonged immune response. Because the data so clearly indicates that two doses of the Pfizer vaccine provide the best protection, he says it’s not worth the risk to stop at one dose.

Looking To the Future

The majority of the vaccine candidates in Phase 3 trials require two doses. Some, however, may be on track to create a single dose option. In September, Johnson & Johnson/Janssen began a Phase 3 trial to test its version of a vaccine that only requires one dose. According to William Moss, MD, executive director at the International Vaccine Access Center at Johns Hopkins University, the company has since begun testing a two-dose regimen.

Due to the potential difficulties of tracking who has received the vaccine and ensuring they return for a second dosage at the appropriate time, Moss said in a press briefing he is hopeful that a single-dose vaccine works through the pipeline.

“The ideal vaccine for COVID-19 would require just a single dose to confer long term protection,” Moss says. “And it’s possible that a vaccine of lower efficacy but with a single dose might do better at a population level than two doses of a higher vaccine if we have trouble getting two doses into people.”

Scientists don’t know much yet about how long the body will be protected from the disease after a person receives two doses of the Pfizer and Moderna vaccines. Future studies may reveal whether people should receive booster shots and at what time intervals, as well as how long immunity to the virus will last.

Further, clinical trial data from Pfizer and the FDA shows only the effect of the vaccine on preventing the development of the disease. There is still a lot to learn about whether a properly vaccinated person can transmit the virus to other people. As new vaccine candidates work through the pipeline, some may be better at preventing transmission than others.

“That’s one of the remaining questions—these studies were not looking at whether vaccination prevents transmission,” Kennedy says. “That’s one additional reason why we need to continue wearing masks—even people who are vaccinated. Until we know the answer, it’d be dangerous not to.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.