From mix-and-match vaccines to Moderna half-doses, what’s new with COVID-19 booster shots?

Oct. 22, 2021
Who should get COVID-19 booster shots. An older woman gets her vaccine at UCHealth Universtiy of Colorado Hospital in Aurora.
Health experts in the U.S. now are encouraging everyone ages 16 and older who is eligible for a booster shot to get it as soon as possible. Photo by Cyrus McCrimmon for UCHealth.

By Katie Kerwin McCrimmon and Erin Emery

(story updated Jan. 6, 2022. For the latest guidance from the CDC on booster shots, click here.)

As omicron infections continue to increase around the world, health experts in the U.S. are recommending COVID-19 vaccines for everyone ages 5 and older and booster doses for people ages 16 and older.

Research shows that COVID-19 vaccines are remarkably effective in preventing hospitalizations and deaths, but the effectiveness of vaccines wanes over time. Booster doses two months after a J & J vaccine or five months after the second dose of Pfizer and six months or longer after two initial doses of Moderna vaccines can jumpstart vaccine efficacy.

Who should get booster shots?


Mike and Nannette Wien pose after getting their COVID-19 booster shots at UCHealth University of Colorado Hospital
Mike and Nannette Wien pose after getting their booster shots at UCHealth University of Colorado Hospital. Mike recently won the Boston Marathon in his age group. He’s eager to encourage everyone to stay healthy and get vaccinated. Photo by Cyrus McCrimmon for UCHealth.

Health experts agree that the best way to end the pandemic as soon as possible is for all eligible unvaccinated people to get their first doses of vaccines as soon as possible. (Learn more about getting COVID-19 vaccines and booster doses.)

Booster shots will help bump up immunities for those who had their vaccines several months ago.

“We know booster shots play an important role in the fight against COVID-19, and we’re still in the midst of a pandemic,” said Dr. Michelle Barron, senior medical director of infection prevention at UCHealth. “Vaccine efficacy may diminish over time with the potential risk for increased susceptibility to breakthrough infections.”

COVID-19 infections and hospitalizations in Colorado remain very high, prompting Barron and others health experts to urge people to get initial vaccines immediately and booster doses as soon as people if they are eligible. Coloradans should continue to be very cautious and wear masks in crowded indoor spaces.

Now that booster shots have been formally authorized for millions of other Americans, we’re providing answers to your key questions on COVID-19 booster shots.

What is a booster shot?

A booster shot is an additional dose of a vaccine after a person has received an earlier dose (or two in the case of COVID-19 mRNA vaccines). An extra dose “boosts” your immune system, sparking better protection against an illness.

Why do we need boosters?

It’s normal for some vaccines to wane or become slightly less effective over time. Research both by the COVID-19 vaccine makers and independent scientists is showing that the mRNA COVID-19 vaccines made by Pfizer and Moderna are waning several months after recipients get their first doses. Also, the delta variant is extremely contagious and it has caused hundreds of thousands of new infections. The omicron variant is also expected to cause more infections in the new year.

What is the specific CDC advice about who should get a booster shot?

The newest guidance from CDC health experts encourages anyone age 12 and older who had their first two doses of Pfizer at least five months ago, or Moderna at least six months ago, or a single dose of the Johnson & Johnson vaccine at least two months ago to get a booster dose as soon as possible.

Joan Hooker, 89, received her vaccine on Sunday at her church, Shorter Community AME in Denver. Who should get booster shots now?
Older adults were the first to get COVID-19 vaccines a year ago and also the first to get booster doses. Now, everyone who is 16 and over and eligible for a booster should get one. Here, Joan Hooker, 90, received her first COVID-19 vaccine dose early in 2021 at her church, Shorter Community AME in Denver. Photo by Cyrus McCrimmon for UCHealth.

When should I get a booster?

If you received a J & J vaccine, you should get a booster shot two months or longer after your vaccine. If you received two doses of Pfizer get a booster after five months; or if you received Moderna get a third dose of least six months after you received your second vaccine doses.

Is the booster dose for Moderna different than the initial Moderna dose?

Yes. Research shows that half a dose of Moderna works well as a booster dose. So, the Moderna booster doses will now be 50 micrograms compared with 100-microgram initial doses.

Have the Pfizer and J & J booster dose amounts changed?

No. People who receive Pfizer or J & J will continue receive the same doses that they previously did.

What’s the difference between a booster dose and a third shot for immunocompromised people?

A third shot is now the standard initial dose for immunocompromised people. These are people who have specific conditions that make it hard for them to build up antibodies to fight infections.

Immunocompromised people should get a third shot about one month after their first two doses of mRNA vaccines like Pfizer and Moderna.

Booster shots, on the other hand, are for everyone else. Healthy vaccinated people should wait two months after a J & J vaccines or five months after their second dose of Pfizer and six months after their second dose of Moderna to get a booster vaccine dose.

Do I have to have a doctor’s order to get a booster shot?

No. Just make an appointment to get a booster dose if you are eligible. No doctor’s order is required. And both vaccines and booster doses are free. Learn more about vaccine locations in Colorado.

How can I get a booster vaccine?

UCHealth patients can log in the online health portal, My Health Connection and can schedule booster doses online. Learn more about COVID-19 vaccines.

How much do booster doses cost?

Booster doses are free, just like the initial COVID-19 vaccine doses.

Can I get a flu shot at the same time I get my booster shot?

woman feeling sick and drinking tea - flu cases could be bad in 2021 and 2022
If you haven’t gotten your flu shot yet, do so right away. Both flu and various strains of the virus that causes COVID-19 are circulating now. Photo: Getty Images.

It’s safe to get flu and COVID-19 vaccines or booster shots at the same time. But, some vaccine clinics only offer COVID-19 vaccines. You may need to schedule a flu shot separately. Please check with your doctor.

If you haven’t gotten your flu shot yet, you should do soon as soon as possible. Flu is here and the omicron variant may cause a new wave of COVID-19 infections, causing a possible “twindemic.” 

Is it true that the Moderna vaccine is staying effective longer than the Pfizer vaccine?

Yes. Research like this study in the New England Journal of Medicine has shown that both the Pfizer and Moderna vaccines are highly effective and very safe. It’s also common for some vaccines to diminish in their effectiveness over time. The Pfizer vaccine seems to be waning (or becoming somewhat less effective) more quickly than the Moderna vaccine.

According to data from the CDC, vaccine effectiveness in preventing hospitalizations for COVID-19 was highest for people who received Moderna vaccines — 93% — compared with efficacy rates of 88% for people who had received Pfizer vaccines and 71% for those who had received the Johnson & Johnson vaccine.

What should people who received the single-dose Johnson & Johnson vaccine do?

About 15 million people in the U.S. received J & J vaccines, far fewer those who have received Moderna and Pfizer vaccines. J & J recipients should get a second vaccine dose two or more months after they got their J & J vaccine. People who received J & J will get better protection if they opt for a Moderna or Pfizer booster dose.

What are ‘mix-and-match’ vaccines? Should I stick with the same vaccine brand that I initially received when I get my booster shot?

Researchers have been studying what happens when people get doses of different COVID-19 vaccines. This is known as getting “mix-and-match” or heterologous vaccines. If you received Pfizer or Moderna and you want to stick with your original brand, you may do so. But, if you want to bump up your immunities with a different type or brand of vaccine, you are welcome to do so. A National Institutes of Health study that included a small number of people found that mixing and matching vaccine types helped increase immunities to the virus that causes COVID-19.

So far, studies are showing that people who received Moderna will get the best protection if they stick with Moderna. People who initially got Pfizer vaccines can bump up their immunities slightly more if they get a Moderna booster. But, the difference is small. It’s most important to get a booster dose as quickly as possible. So, get the dose you can most easily and quickly find.

Learn more about mix-and-match boosters. Read a recent study about mix-and-match boosters.

What if I am vaccinated and got a breakthrough case of COVID-19? How long should I wait to get a booster dose?

You should wait until you have recovered from COVID-19, no longer have symptoms including a fever and no longer are in isolation: about 10 days for most people and about 20 days if you are immunocompromised.

In general, people should get their booster doses as soon as they are healthy and qualify for the booster, says Barron, the UCHealth infectious disease specialist.

“It is thought that people potentially should wait up to 90 days after their infection to get a shot. But, no one has studied the ideal time frame,” Barron said. “We recommended that if you have fully recovered from COVID-19, you feel well and are no longer in isolation, you should get your COVID-19 booster shot whenever it’s convenient for you.”

I had a breakthrough case of COVID-19 and needed monoclonal antibodies? How long should I wait to get my booster shot after having received monoclonal antibodies?

People who have received monoclonal antibodies should wait 90 days before getting their booster dose.

Are there enough vaccines available in the U.S. for people to get both their initial doses of vaccines and their booster doses?

Yes. CDC health experts say that vaccines are plentiful. So, supply is not a problem. Get your initial vaccines immediately if you have not already. And, if you are 16 or older and eligible for a booster dose, get it as soon as possible.

Who is considered immunocompromised?

  • Cancer patients.
  • Organ and stem cell transplant patients.
  • People with immunodeficiencies.
  • People living with HIV.
  • Patients who are being treated with immunosuppressive medications such as chemotherapy, TNF blockers to stop inflammation tied to rheumatoid arthritis, certain biologic agents like rituximab and high-dose corticosteroids.

Are boosters recommended because of breakthrough cases, COVID-19 infections in fully-vaccinated people?

  • Increases in the number of breakthrough cases of COVID-19 have caused health experts to recommend booster shots for some people.
  • But, most people getting sick with COVID-19 now are unvaccinated. And, an overwhelming majority of hospitalized COVID-19 patients have not been vaccinated.
  • Some immunocompromised people have been getting COVID-19 even if they are fully vaccinated. That’s because their bodies are unable to create the same number of antibodies as healthy people, and thus, they cannot mount the same defense against COVID-19.
  • In particular, people with cancer, rheumatoid arthritis, lupus and patients who have had organ or bone marrow transplants and those who are taking immunosuppressive medications have not been able to build up the same level of antibodies to COVID-19 as people without underlying health conditions.

How long are coronavirus vaccines effective?

Both the Moderna and the Pfizer vaccines — which account for more than 95% of U.S. vaccinations so far — remain highly effective for five to six months after people receive their second dose. The efficacy data is based on studies of how clinical trial participants have fared over time. The efficacy has declined slightly over the summer and fall, both because of the delta variant and the waning effect.

What are the side effects of the third dose?

So far side effects for boosters are similar to those that people experienced when they got their first two doses.

Individuals may experience a sore arm, headache, muscle aches, a low-grade fever or feel tired. These side effects typically last fewer than three days. Experts from Pfizer told FDA and CDC officials during recent testimony that many people receiving booster doses have experienced fewer side effects after third doses than they did with their second dose.

Who should skip booster doses?

Young, fully-vaccinated, healthy people probably don’t need booster doses because the vaccines are working very well to protect them from severe infections, hospitalizations and death from COVID-19, according to CDC experts.

Because the vaccines are holding up so well for young, healthy people, some infectious disease experts were hesitant to recommend booster doses for all adults.

In addition, in very rare cases, young men who have been vaccinated have experienced heart issues known as myocarditis. Due to this very rare vaccine side effect, some younger men, ages 18 to 30, may decide to skip booster doses.

Are people in other countries getting booster doses?

Yes. Israel has led the way. In Israel, older adults began getting booster doses in the early summer and now, anyone who is 12 or older can get a booster dose. Other countries like the United Kingdom and Germany also are offering booster doses.

Is there a test to determine how strong your immunity is against COVID-19?

Yes, there are antibody tests. But, doctors do not recommend antibody testing outside of clinical trials. The best way to stay healthy is to get your primary COVID-19 vaccines as soon as possible, then to get a booster dose if you qualify or fall into one of the recommended groups.

If I had COVID-19 already, do I still need vaccines?

Yes. Studies like this one are finding that vaccines are even more protective than natural antibodies. And, people can get COVID-19 after having previously had it. So, it’s best to get fully vaccinated.

Are antibodies from vaccines or previous infections the only mechanisms in our body that are fighting COVID-19 infections?

No. Researchers are finding that antibodies from vaccines team up with natural “memory” cells in our bodies. These are known as “B” and “T” cells.  CDC researchers estimate that antibodies play a majority role in fighting COVID-19 infections, but “B” and “T” cells are also crucial.

I have Multiple Sclerosis (MS). I need to get infusions every six months. My medication is called Rituxan. Do I need to wait for a couple of months after getting my infusion to get my booster shot?

No. COVID-19 infections can be dangerous for people with conditions like MS.

“While Rituxan can decrease the efficacy of the vaccine, with rates of COVID-19 still high, most clinicians recommend not waiting to get the booster shot if you are eligible,” Barron said. “The protection may not be as good as if you wait, but then you are left unprotected for the time frame until you can get it and you are at risk of infection. So the short answer is: If you qualify for a booster, you should just get it regardless of when the Rituxan dose was given.”

About the author

Katie Kerwin McCrimmon is a proud Coloradan. She attended Colorado College thanks to a merit scholarship from the Boettcher Foundation and worked as a park ranger in Rocky Mountain National Park during summers in college.

Katie is a dedicated storyteller who loves getting to know UCHealth patients and providers and sharing their inspiring stories.

Katie spent years working as an award-winning journalist at the Rocky Mountain News and at an online health policy news site before joining UCHealth in 2017.

Katie and her husband, Cyrus — a Pulitzer Prize-winning photographer — have three adult children and love spending time in the Colorado mountains and traveling around the world.