Since it was approved for use, the Johnson & Johnson COVID-19 vaccine has been marketed with one major advantage over its competitors: it only requires one dose. With the exponential spread of the highly transmissible delta variant, however, concerns have arisen about whether recipients will need to “top off” their immunity against the now-dominant strain in the United States with a second shot.

Although the J&J vaccine was less effective in clinical trials at protecting against symptomatic COVID-19 than the two-dose mRNA vaccines from Pfizer and Moderna, it was 100 percent effective against hospitalization and death.

Recent research indicates that mRNA vaccines, such as the Pfizer and Moderna, appear to be effective against the delta variant if fully vaccinated with two doses.

Pfizer reported Thursday that preliminary data indicated that receiving a third dose six to 12 months after being fully vaccinated may provide additional protection, and that it would have data to publish in the coming week in order to ask the Food and Drug Administration and the Centers for Disease Control and Prevention to recommend a third dose.

Johnson & Johnson has released data indicating that their single dose is also effective against the dominant variant, but it was a small study, and people are assuming that more is more. Preliminary evidence suggests that the J&J vaccine provides adequate protection against the delta variant.

In a laboratory experiment, researchers examined the blood of ten people who had received the Johnson & Johnson vaccine and tested it against a number of concerning variants, including delta.

They discovered that the vaccine appeared to be effective against the variants, as evidenced by “neutralizing antibody titers” and other immune system response indicators.

The level of immune response required to achieve protection is still unknown, but the experiment provided promising hints, if not conclusive proof of real-world protection. The news provides some reassurance to the more than 12 million Americans who have received the J&J vaccine; however, it has prompted some medical experts to reconsider the role of a “booster” shot — whether it will still be required, and if so, when and for whom.

So far, all vaccines available in the United States have been shown to be both safe and effective for at least eight months. Brownstein believes that receiving a booster is likely safe and looks forward to more information. In the meantime, this new data gives those who received the J&J vaccine some hope that they are still protected for the time being. “We want to wait for data to support any kind of vaccine strategy that we recommend,” Brownstein said, highlighting the importance of uniform recommendations backed by data.

The science has yet to be settled, experts say.

“I think there are still open questions for those who are immunocompromised,” Brownstein said. “Talk to your doctor — especially if you’re in the high-risk category.”

Dr. William Schaffner, professor of medicine in the division of infectious diseases at Vanderbilt University School of Medicine, explained that “when talking about immunocompromised people, that includes a wide range of disease, and there are no studies at the moment to indicate that an additional dose will benefit you or what kind of reactions you might get from an additional dose.”

For the time being, Schaffner is adamantly opposed to the idea of an additional booster shot in the absence of formal recommendations. He emphasized that the CDC has yet to recommend any booster under any circumstances. Following Pfizer’s announcement, the FDA and the CDC issued a joint statement urging caution on the need for a booster shot, citing the fact that only 47.7 percent of the U.S. population is fully vaccinated at this time.

While many experts remain divided on the issue of boosters, this is not due to a lack of faith in the single-shot vaccine’s ability to do its job. Dr. Vin Gupta, a critical care pulmonologist and faculty member at the University of Wisconsin Medicine, believes that an additional booster shot on top of the single J&J vaccine would provide an extra layer of protection. Gupta advocated for a booster not because he thinks the J&J shot doesn’t work well, but rather, to ensure level protection for all vaccines across the board. He stands by that stance.