With three COVID-19 vaccines approved in the U.S. and more possibly on the way, many people are wondering which vaccine is right for them — or whether mixing and matching the shots may provide better long-term protection.

The most recently approved vaccine, from Johnson & Johnson, has lower efficacy numbers than the Pfizer-BioNTech and Moderna shots. Given the data, some people have questioned whether they can get a different COVID-19 vaccine down the line if they’re dissatisfied with a shot already received.

Health experts discourage comparing vaccines based on efficacy data alone, and say that mixing and matching shots isn’t recommended until more data is available.

It’s difficult to accurately compare vaccines.

In clinical trials, the Johnson & Johnson vaccine was found to be 66.9% effective at preventing moderate to severe or critical COVID-19 illness after at least 14 days and 66.1% effective at preventing those levels of illness after at least 28 days.

It was also found to be about 77% effective in preventing severe or critical cases of COVID-19 at least 14 days after vaccination and 85% effective in preventing severe or critical cases at least 28 days after vaccination.

Those numbers are lower than seen in the two-dose Pfizer vaccine, which was 95% effective at preventing COVID-19 at least seven days after the second dose, and the two-dose Moderna vaccine, which was 94.1% effective at preventing COVID-19 at least 14 days after the second shot.

But comparing those numbers alone is “like comparing apples to oranges,” said Katelyn Jetelina, an epidemiologist and assistant professor at the UTHealth School of Public Health in Dallas.

“Those studies were very different,” she said. “It’s really not a fair comparison.”

Health experts say the three vaccines measured efficacy differently during clinical trials, making those numbers alone hard to compare. Pfizer and Moderna focused primarily on finding out whether their vaccines prevented a symptomatic COVID-19 infection, while Johnson & Johnson primarily analyzed how well its vaccine prevented moderate to severe illness, which was defined differently.

Significant protection

All three vaccines provide significant protection against the more serious outcomes of a COVID-19 infection, health experts say.

“Even if you do want to compare those, efficacy isn’t the right comparator. We need to compare number of hospitalizations, severe disease and deaths,” Jetelina said. “Johnson & Johnson does a fantastic job of reducing severe disease, hospitalizations and deaths, almost exactly to Pfizer and Moderna.”

Health experts also say it’s important to recognize that the vaccines were tested at different times. Moderna and Pfizer’s shots were tested last year, before variants of the virus were widely circulating.

All three vaccines still far exceed the 50% efficacy target set by the World Health Organization last April, when manufacturers started exploring and developing COVID-19 vaccines.

And all three are also far more effective than influenza vaccines, which millions of Americans get annually and which are only estimated to reduce the risk of flu illness by 40% to 60% each year.

Dr. Beth Kassanoff, an internal medicine physician with North Texas Preferred Health Partners and president of the Dallas County Medical Society, said the most important thing to note is that the data shows that people don’t need to be picky about which vaccine they get.

“Getting the Johnson & Johnson vaccine, you’re not getting an inferior product,” she said. “You’re not signing up for the second best. You really are getting an excellent, excellent vaccine.”

Although she received a two-dose shot, she said she’d “feel fully confident” in recommending the Johnson & Johnson shot to her family members.

“If they weren’t [vaccinated] and they got offered the single dose J&J one, I would say take it in a heartbeat,” she said.

Mix and match?

To achieve maximum immunity, people who get two-dose vaccines should receive their doses from the same manufacturer each time during the recommended time frame.

Not much is known about the efficacy or protection that would be provided if someone were to combine a dose of the Moderna vaccine with a dose of the Pfizer vaccine. Only in “exceptional situations” should providers give patients different shots for each dose, according to the Centers for Disease Control and Prevention.

The CDC defines an exceptional situation as one in which the manufacturer of a person’s first dose can’t be determined or is no longer available beyond a temporary period. In that case, a person could get one dose of Moderna and one dose of Pfizer.

Also, little is known about how different types of vaccines interact with each other.

The Pfizer and Moderna shots are both mRNA vaccines. They work by using a genetic code that teaches the body how to make a protein, which then sets off an immune response to fight the virus.

The Johnson & Johnson shot, on the other hand, employs a more established technique. The vaccine uses a modified adenovirus, a common virus that causes cold and flu-like symptoms, to include the genetic information for a spike protein found on the virus that causes COVID-19. Once the body makes replicas of the spike protein, the immune system can use them to respond to the real virus.

It’s not known how mRNA vaccines will interact with adenovirus shots. The CDC says that in “limited, exceptional situations” where people are unable to complete the two-dose shots (for example, because they had an allergic reaction to their first dose), they can get the Johnson & Johnson vaccine after. In that case they would be considered to have received a valid, one-dose vaccine, not a mixed-dose regimen, the CDC says.

Stick to studies

For people who get the Johnson & Johnson shot and want to get a two-dose vaccine when supply is no longer limited, Kassanoff said health experts think it wouldn’t be harmful, but there isn’t enough data to know for sure whether that’s true.

“If you got the Johnson & Johnson and there were all the supply in the world, and you said, ‘Well, I just want to make sure that I’m covering all my bases,’ it wouldn’t hurt you, we don’t think,” she said. “We don’t think there’s any medical reason it would be harmful to get the other one in the future.”

In the United Kingdom, studies are underway to test the efficacy of receiving the Pfizer vaccine with a vaccine developed by Oxford University and AstraZeneca, which also uses an adenovirus.

Health experts say for now, people should stick to taking the vaccines as they were studied. It’s “way too early” to be thinking about booster shots or mixing doses, Jetelina said.

“We need to shy away from mixing the vaccines and just have people stick to their regimen for now,” she said.

Twitter: @catherinemarfin