In a study, J&J's vaccine produced fewer antibodies to the Delta variant compared with other shots.
The study authors said the lower antibody response "could result in decreased protection."
But other experts said the COVID-19 lab-based study didn't represent the real world.
A lab experiment showed Johnson & Johnson's single-dose vaccine produced a weaker antibody response against the Delta variant when compared with Moderna's and Pfizer's double-dose shots - but it will probably still work against the variant in real life, experts say.
New York University researchers drew blood from eight people who received Moderna's vaccine, nine people who got Pfizer's, and 10 people that got J&J's, according to a preprint version of the study posted Tuesday. They compared the antibody response against Delta with the antibody response against the original strain of the coronavirus.
In the Moderna and Pfizer group, the antibody response was three times lower against Delta on average. For J&J's shot, it was 5.4 times lower against Delta, the study authors said.
The study authors said the lower antibody response for J&J's shot "could result in decreased protection." More than 9 million Americans have received that vaccine.
The Delta coronavirus variant, which is the most common cause of new infections in the US, is about 50% more infectious than the formerly dominant Alpha variant and has mutations that can help it avoid the immune response.
Dr. Ned Landau, who led the experiment, told CNBC that the findings suggested people who got the J&J vaccine "should at least consider" a second dose of the same vaccine or one from Pfizer or Moderna.
But other experts aren't convinced about the findings of a small lab study, which hasn't yet been scrutinized by other experts in a peer review. They say Johnson & Johnson's vaccine could still work against Delta in real life.
Insider's Hilary Brueck reported Tuesday that fully vaccinated people could get COVID-19 - but if they do, they usually get mild symptoms, or none at all.
Eric Topol, a professor of Molecular Medicine at the Scripps Research Institute, said on Twitter Tuesday that the antibody response with J&J's vaccine was above the threshold "for concern."
"There's also the T cell response," he added. The T-cell response is another aspect of the immune system - it is harder to study in the lab but is thought to be crucial to protect against variants. The NYU team didn't examine this in their study.
Peter Chin-Hong, a professor of infectious disease at University of California, San Francisco, told ABC10 News, "You can't necessarily extrapolate laboratory-based studies to what happens in real life." He cited J&J's performance against the Beta variant.
The same NYU study showed that the J&J vaccine's antibody response against Beta variant, which was first found in South Africa, was 6.5 times lower than its response against the original variant. But in humans, J&J's vaccine was 64% effective at preventing moderate to severe disease in its South Africa trials, when 95% infections were caused by the Beta variant.
Real-world data from South Africa, posted by the South African Medical Research Council on July 1, showed that 94% of health workers who were vaccinated with J&J's shot and then caught COVID-19 had only mild infections.
The company said on July 2 that its COVID-19 vaccine should work against Delta.
Despite this, some experts who received J&J shots have opted to have an extra dose of Pfizer's or Moderna's vaccine.
Neither the Centers for Disease Control and Prevention nor the Food and Drug Administration recommend that people who received the J&J shot take an extra dose. There isn't enough data to support the approach, they say.
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