What to know about booster shots and third doses of the COVID-19 vaccine

Booster doses of the COVID-19 vaccines are now widely available, and adults (and some teenagers) are being encouraged to get the extra dose of the vaccine.

The U.S. Centers for Disease Control & Prevention recommends a booster dose of the Pfizer vaccine for people 16 and up if it’s been six months since their second dose of COVID-19 vaccine. Booster doses of the Moderna vaccine are also available for adults 18 and over who had their second dose of vaccine at least six months ago. And public health officials say anyone who got the one-dose Johnson & Johnson vaccine should get a booster after two months. (Those who got Johnson & Johnson can get another J&J dose or a single dose of either mRNA vaccine. In fact, it’s officially okay to mix and match any of the vaccines so there is no need to look for any specific brand when it’s time to get boosted.)

Unlike healthy adults whose vaccine protection was beginning to wane after six months, a third dose has long been encouraged for people who are immunocompromised. That’s because research showed many people who had weakened immune systems didn’t receive full protection from their COVID-19 vaccines. That could leave them particularly vulnerable to COVID-19, especially as more contagious variants circulate in a community. In this case, the third dose is considered part of the initial series and is best given eight weeks after the second dose.

Whether you’re someone who’s healthy and ready for a booster, or you’re someone who’s immunocompromised and in need of a third dose, these extra rounds of vaccines will provide additional immune system support. Without them, people may be more susceptible to COVID-19, which is especially worrisome as new variants — like Omicron — emerge and circulate widely.

As an infectious diseases specialist and hospital epidemiologist at the University of Chicago Medicine, I think it’s critical that people understand why this extra dose is needed.

Am I eligible for a third dose or a booster dose of the COVID-19 vaccine?

If you’re over the age of 16 and it’s been six months since your last mRNA vaccine, then yes, you’re eligible to get a booster dose. Meanwhile, if it’s been two months since your Johnson & Johnson vaccine, you’re also eligible for a booster. In addition, a third dose is also encouraged for people with weakened immune systems along with people who are 65 and older and those who live or work in certain high-risk settings as soon as 8 weeks after the second dose. While the criteria will most likely change, this group currently includes patients who’ve had or are receiving:

• Organ transplants
• Stem cell transplants within the past two years
• Active cancer treatment for tumors or blood cancer and are undergoing chemotherapy that affects the immune system
• Severe primary immunodeficiency
• Advanced or untreated HIV
• Active treatment with high-dose corticosteroids or other drugs that may suppress your immune response.

Is a third dose the same thing as a booster dose?

The actual vaccine will be the same, but the terminology is different. A booster is given to people who got a full course of a vaccine and developed a good response, but whose initially strong immune response has started to decrease (or wane). When that happens, people are offered booster doses to pump their immune response back to previous levels. In the case of Moderna, the booster dose is only half of the original dose.

Unlike boosters, third/additional doses of COVID-19 vaccines are for people who received the complete starter series of vaccines but then their immune systems didn’t have a good enough response. Evidence shows these are generally people whose immune systems are weaker. That’s why the FDA and CDC first recommended an additional dose for immunocompromised individuals. Those needing a third dose of Moderna should get the full dose.

Does my booster vaccine or third dose need to be the same brand as my initial vaccine?

With one notable exception, the CDC says booster doses don’t need to match the original vaccines people received earlier this year. The choice is yours, but the CDC has recently recommended you should opt for the mRNA vaccines from Moderna or Pfizer when they’re available instead of the Johnson & Johnson vaccine. That recommendation applies even if you received J&J as your initial vaccine.

The lone exception at this time is 16- and 17-year-olds who received the Pfizer vaccine. They should only receive a booster dose of Pfizer.

If I am immunocompromised, will I be fully protected after I get my third dose of the vaccine?

It depends. If you are immunocompromised, a third dose is supposed to provide you with better protection from COVID-19, but it may not provide you with the same level of immunity as healthy people. In studies, most participants who had any immune response to the first two doses did better after a third dose, but some people didn’t. That means you should still take extra precautions to avoid getting COVID-19, such as wearing a mask (especially inside) and avoiding large crowds or high-risk activities. Consider wearing eye protection (like a face shield), particularly if you’re around unmasked people or in high risk, crowded settings.

Discuss your ongoing risk with your healthcare provider and ask them how you should reduce your risk. You may need additional doses beyond the first three. It’s important that your family members and regular close contacts get vaccinated, too. This decreases the likelihood that you will be exposed to COVID-19.

Can children get a booster dose yet?

Vaccines for children 12 and up began this spring, and children 5 and up were able to get COVID-19 vaccines this fall. However, it may be months before children under 5 can get vaccinated. Research remains underway to determine when children should get a booster dose.

Why didn’t we know third doses would be needed before?

We’re still learning about the COVID-19 vaccines and how well — and for how long — they protect us from the virus and its emerging variants. We will keep learning about how the vaccines protect us from emerging variants in the coming months and years, which means we will adjust our vaccine plan accordingly.

Will more people keep needing additional doses of the COVID-19 vaccines?

It’s almost certain that most people will eventually need to get additional doses of the vaccines. However, we are still studying how long the vaccines will protect people. The good news is that booster shots/third doses are widely available right now. Data show they provide additional protection for people, particularly against new variants. If you haven’t gotten your booster yet — or even been vaccinated at all — you should get yours right away.

Why are people getting breakthrough infections if the vaccines are effective?

COVID-19 vaccines create high levels of antibodies that can block the virus from ever infecting our cells. As time passes after your vaccination, however, you also develop memory B cells and T cell immunity and antibody levels go down. With fewer blocking antibodies, the virus might be able to start an infection. As viruses evolve, strains that can bypass those antibodies have an advantage and some people have such a high-level exposure that it can overwhelm the antibodies they do have. When this happens, we call it a “breakthrough infection,” but memory B cells and T cells are able to respond quickly and stop the infection before too much damage is done.

When otherwise healthy vaccinated individuals develop breakthrough infection, it is usually mild. Unvaccinated people don’t have existing antibodies or memory B cells or T cells waiting to fight off COVID-19, which means they have to start their immune response from scratch. This means that much more damage is done to their organs and tissues before COVID-19 is killed off, which can lead to complications like having low oxygen levels, as well as lung, kidney and heart damage. Unvaccinated individuals are much more likely to need intensive care support or have lingering symptoms known as long-COVID.