Scientists gave J&J vaccine recipients different booster shots in a highly anticipated trial, and found they got a much better immune response with Moderna and Pfizer

• People who got the J&J vaccine may get a better immune response from a Moderna or Pfizer booster, a major new study suggests.
• J&J recipients generated far more antibodies after a Moderna or Pfizer shot, instead of a second J&J jab.

The first US study to mix and match boosters of Moderna, Pfizer, and Johnson & Johnson's COVID-19 shots is out, and it looks like you can boost any US-authorized vaccination with any other vaccine safely - though boosting J&J with Moderna or Pfizer may prompt a stronger immune response, at least initially.

The new study, funded by the US National Institutes of Health, showed first and foremost that mixing and matching COVID-19 vaccines is a perfectly safe thing to do.

"What the study shows is that regardless of what an individual received originally, getting boosted with one of the three vaccines that we evaluated, the one from Moderna, the one from Janssen, the one from Pfizer, led to good antibody responses in each of the groups," lead study author Dr. Robert Atmar from Baylor College of Medicine told Insider, shortly after his new data was released on Wednesday.

The mix-and-match study enrolled 458 people from 10 different medical centers across the US who were each fully vaccinated with Moderna's, Pfizer's, or J&J's [Janssen] vaccine. Volunteers then got boosted with one of those three shots, yielding nine different mix and match combinations. Researchers tested the blood of those volunteers periodically throughout the next month, comparing their levels of virus-fighting proteins called neutralizing antibodies.

The study provides some of the clearest evidence yet that all the booster shots - Pfizer, Moderna, and J&J - increase antibody levels, but that a booster from Moderna's or Pfizer's vaccine yields a stronger response than another J&J jab. (It's similar to what researchers in Europe have found boosting AstraZeneca's vaccine with Pfizer.)

People who got their first shots from J&J saw the strongest responses from Pfizer or Moderna boosts

The clearest improvement in neutralizing antibody responses - which are an imperfect, but easy-to-test proxy for measuring initial vaccine-induced immunity - came from people who initially got J&J's vaccine.

In this group, a J&J booster shot raised neutralizing antibody levels by 4.2-fold on average. But J&J recipients who got a Moderna shot instead saw a 76-fold increase in antibodies, and a Pfizer booster yielded a 35-fold jump. The differences between these groups were statistically meaningful, meaning it's highly unlikely they are a product of chance.

The researchers warn that with only roughly 50 people in each group, this study is too small to fairly compare different vaccine booster combinations side by side, and the trial's limited follow-up time (of just one month) doesn't tell us how durable each booster's protection may be in the long run.

Atmar cautioned that this study was "not designed to really make comparisons between different groups," but instead "to provide data rapidly for public health decisions."

Still, he acknowledged, "the natural thing" people do "is to want to make comparisons." And the tables show stark differences:

The same trends above held true for neutralizing antibody titers, a more durable (though still not perfect) picture of vaccine-induced immunity.

Antibodies are not everything when it comes to immunity

Atmar cautioned that this does not mean we all need mRNA boosters.

"I don't think we're going to want to end up boosting people every six months," he said.

While antibodies are a key part of the body's initial immune response, there are other long-term components of immune memory, like the cellular immune response, which weren't measured in this study.

As far as side effects are concerned, by far the most common complaint post-booster was some mild arm pain, which more than 70% of patients in all mix and match groups experienced.

Health regulators will vote on boosters this week and next

The highly-anticipated study is some of the most compelling research on the safety and immunogenicity of boosters. And it comes at a critical time, too: An FDA expert panel will meet Thursday and Friday to discuss Moderna and J&J boosters, while a CDC panel convenes on the same issue next week. (Third doses of Pfizer were OK'd by both last month.)

J&J's booster shot application has already fallen under FDA scrutiny, with the agency's scientists highlighting a lack of robust clinical trial results supporting a booster at six months.