Why it's important to get the second dose of the COVID-19 vaccine

Experts explain why it's essential to get both doses of the Pfizer and Moderna vaccines.

Jessica Rendall Wellness Writer
Jessica is a writer on the Wellness team with a focus on health news. Before CNET, she worked in local journalism covering public health issues, business and music.
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Jessica Rendall
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Don't skip your second vaccine dose -- it's important.

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The COVID-19 vaccines available in the US are extremely effective, and with three that offer 66% to 95% overall efficacy, and even higher protection against severe illness and death, there's reason to celebrate. For comparison, the average flu vaccine is around 40% to 60% effective.

But this potency hinges on the fact we take the vaccine as directed, which means both doses of either the Moderna or Pfizer shot. (Johnson & Johnson only requires one dose.) Although the majority of people show up for their second appointment, some have missed the second dose -- around 8%, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said at a recent White House press briefing.

In the briefing, Fauci pointed out data from different studies showing that though one dose may be enough to offer a little protection, completing the vaccine series and getting both doses of the mRNA regimen is the best way to ensure protection against COVID-19.

Studies show the first dose gives some immunity. Should you settle?

No, says Dr. Anne Liu, infectious disease physician at Stanford Health, because the protection given by the first dose was measured in the short term, and data is limited. "It's true that with the first dose there seems to be a pretty substantial degree of protection, starting a few weeks after the first dose," Liu said. "We don't know how long that protection lasts." 

Another reason we can't put full faith in the first dose is the fact that immunity can be difficult to measure, according to Liu. She says it's much easier to measure an antibody response (think blood test, or vaccine side effect response), but that it doesn't show the "full picture." Scientists look at antibody responses because they're easier to measure than cell activity, which is more complicated, Liu says. 

Immune responses vary person to person, and there are health conditions, like having a blood-based cancer or being a transplant recipient, that have impaired vaccine response, Liu says.

Getting the second dose also sends a powerful reminder for your immune system to attack when necessary. 

"The purpose behind the second dose is to make the antibody that is produced more specific," Liu says. "To refine the memory response, and also to boost the levels of antibodies."

I've waited too long and missed my second appointment. What now?

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Getting the second shot later is definitely better than never, according to the Centers for Disease Control and Prevention, and getting the booster shot late or early doesn't mean you have to restart the vaccine regimen. The recommended time frame is getting your second dose of Pfizer 21 days after the first shot, and Moderna's second dose 28 days after the first. The CDC also states that you can get the booster shot up to six weeks after the first dose. Either way, getting the second shot late will not hurt you. 

For those worried about being late to the second-dose game, Dr. Hana Akselrod, assistant professor of medicine at George Washington University, says to keep in mind the vaccine series children receive. 

"The comparison I would make is childhood vaccines," Akselrod says. "When something gets in the way and a child misses what should have been a three-month or six-month or one-year follow-up, the pediatrician doesn't give up on vaccinating them. They have them scheduled at the next available opportunity." 

Can I mix and match vaccine doses?

As of now, the advice is to stick with the same brand for both vaccine doses. Dr. Aditya Shah, infectious disease consultant at the Mayo Clinic in Minnesota, says there isn't enough data to give people the green light to switch vaccines just yet. "I don't have enough data to say otherwise," Shah says. "It should be OK, but we need more data."

However, you might not have to travel to the same site or pharmacy for your second dose of the vaccine, thanks to programs like the Federal Retail Pharmacy Program, which is a collaboration between national and local pharmacies to help harder-to-reach populations or those who live in rural areas. 

Why do mRNA vaccines require two doses? 

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Johnson & Johnson's viral vector vaccine, back on the market after the CDC and the Food and Drug Administration issued a pause to investigate rare cases of blood clotting in women, only requires one dose for a person to be considered fully vaccinated. The makeup of this vaccine, similar to Europe's AstraZeneca, is different from the Pfizer or Moderna vaccine, which uses mRNA technology. Without getting deep into the technological differences in the vaccines, Akselrod says that research done in the clinical trials determines what's necessary in terms of vaccine dose and administration. 

What scientists look for in any of the COVID-19 vaccines, Akselrod says, is a robust immune response that signifies lasting protection and immune system memory. 

"It's important to get both doses of the Pfizer and Moderna vaccines as developed, as tested, as scheduled," she says.

Watch this: Stanford expert's bottom line on what a COVID vaccine means for you

What about COVID-19 variants? 

Getting both doses of the Pfizer or Moderna vaccine also ensures you have the best protection possible against variants of COVID-19, particularly the mutation that is causing devastation in India. Even as the virus mutates, changes and finds ways around a vaccine that was tested with earlier variants, Akselrod says, the hope is that full vaccination will prevent the most severe cases of COVID-19 and prevent death. 

"We're in an arms race with this virus right now, with regard to the variants," Akselrod says.

An underreported issue involving COVID-19 variants, according to Liu, is the way the virus can mutate in immunocompromised patients, because of illness or certain drugs they may be on that change immune function. Because the virus is still alive in their bodies (this is different from Long COVID, which gives patients lasting side effects with no active virus), the virus in these patients will survive and develop mutations, Liu says. 

If we don't fully vaccinate enough people with healthy immune systems, Liu says, the virus will have more space to mutate and spread. 

What can we do to make it easier for people to get both doses?

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"We need to do a lot better at educating people about the second shot, and providing avenues for them to come back for the second shot," Shah says. 

Outreach programs like the Federal Retail Pharmacy Program and losing the appointment requirement at vaccination sites is a start, but Liu says it's also important to understand why people may be hesitant to get the second dose, or any dose at all. 

"For people who are not getting the second dose, reach out to your doctor to discuss it before you make your final decision," Liu says. "Talk to people who have your best interests in mind." 

One reason some people might be hesitant to go in for the second dose is nerves over side effects which, Shah emphasizes, will be very temporary.

Another reason to get both doses, Akselrod argues, is that you want a full return on what's arguably your most precious investment, your health. 

"I would recommend to people, both in my personal life and in my professional capacity, not to cut corners when it comes to their health," she says. 

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The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.