Two weeks ago, virologist Angela Rasmussen received a dose of the Pfizer-BioNTech coronavirus vaccine to boost her immune system, which was already primed by a Johnson & Johnson shot.

No U.S. health agency has recommended this vaccine combo. And Rasmussen, a research scientist at the University of Saskatchewan’s Vaccine and Infectious Disease Organization in Canada, remains confident in data that show one J&J dose will prevent her from getting hospitalized with covid-19, the illness caused by the virus.

But she was concerned about a rare post-vaccination breakthrough infection — though not because she worried it would make her sick. Instead, if she were exposed and her immune system did not roundly defeat the infection, she feared any surviving pathogens may have the opportunity to evolve into more resistant strains. The emergence of the delta variant, a version of the coronavirus that more easily spreads from person to person, troubled Rasmussen. The Pfizer shot, she said, could reinforce her protection against that variant or help stop her from spreading it.

Rasmussen received one of the 12 million doses of the Johnson & Johnson vaccine administered in the United States so far. Meanwhile, 130 million Moderna and 175 million Pfizer shots had been administered to people in the United States by mid-June, according to the Centers for Disease Control and Prevention. Numerous studies have shown that the Pfizer and Moderna vaccines, which belong to a class of vaccines that use molecules called mRNA, are effective against delta and other variants. Fewer studies address J&J’s strength against variants.

As new coronavirus variants emerge and the longevity of vaccine protection remains unknown, scientists are researching how booster shots could work. (The Washington Post)

Because more scientific reports involve mRNA vaccines rather than J&J vaccines, Rasmussen said she thinks people who received the J&J shot “are feeling left behind.” Some of those people may be wondering whether they, too, need boosters.

Rasmussen’s circumstances are unusual — she was able to secure an mRNA dose in Canada, where she moved weeks after getting the J&J shot. Canada’s vaccine advisory committee supports following the AstraZeneca vaccine with an mRNA vaccine. J&J and AstraZeneca use a related type of vaccine technology; they’re both adenovirus vector vaccines, though the AstraZeneca vaccine is not authorized in the United States.

“I’m a virologist who works at a vaccine research institute, so I didn’t feel that I needed to discuss anything with the person who administered my vaccine shot,” Rasmussen said. “I made a professional evaluation of my own situation.”

The average J&J recipient has neither her expertise nor her access. Many U.S. vaccine and infectious-disease specialists who spoke with The Washington Post cautioned against attempts to find boosters unless supporting data or an official recommendation emerge. Members of the public vaccinated with J&J, they argued, should not independently seek out extra doses. Here are three main questions to consider.

Frequently Asked Questions

  • Is it necessary to combine the Johnson vaccine with another vaccine?
  • Is it safe to boost the Johnson vaccine with an mRNA dose?
  • Is it even possible to access a second shot if you’ve already had a Johnson dose?

Is it necessary to combine the Johnson vaccine with another vaccine?

The J&J vaccine, like the other two Food and Drug Administration-authorized vaccines, is robustly protective against severe disease, hospitalization and death.

“If you look at people who are being hospitalized now, it’s really been those persons who have not been previously vaccinated,” said Robert Atmar, a professor of infectious diseases at Baylor College of Medicine in Houston. “From a public health standpoint, what we need to do, in my opinion, is try and get our populations who aren’t vaccinated to accept a vaccine that’s readily available, and that will have a much larger impact than trying to roll out a booster strategy at this point.”

The three U.S.-authorized vaccines provoke immune systems in similar ways. Each alerts immune fighters to SARS-CoV-2, the virus that causes covid-19, by exposing the body to the virus’s spike protein, said Dan H. Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston, whose laboratory helped develop the J&J shot. While it’s likely that adding a booster to any of the available vaccines will increase immune responses, “how that translates into clinical protection, that’s the part that’s not yet known,” Barouch said.

Whether booster shots are necessary largely depends on how effective the three authorized vaccines are against the circulating variants, particularly the highly transmissible delta variant, and how long a fully vaccinated individual is protected.

A growing body of evidence suggests the coronavirus vaccines developed by Pfizer-BioNTech and Moderna continue to protect against the delta variant with only modest decreases in effectiveness. Another recent study found that the two mRNA vaccines can trigger a persistent immune response that may lead to long-lasting protection.

A small study released Thursday by Johnson & Johnson, which has not yet gone through peer review, found that protective immune responses persisted for the entire eight-month study period in 10 people who had one J&J dose. That study measured the levels of immune fighters in the subjects’ blood: neutralizing antibodies, which bind to viruses and keep the pathogens from infecting healthy cells, as well as T cells, which recognize and obliterate infected cells.

A single J&J shot produced neutralizing antibodies against delta and other common variants of concern, including the beta variant first identified in South Africa. And the overall amounts of those antibodies declined only slightly in eight months of study, said Barouch, who was a lead author of the new study. This shows “very good durability of the vaccine.”

The bottom line, Barouch said, should be reassuring: “There’s robust neutralizing antibody responses and good cross-reactivity against all the variants, including delta.”

High rates of vaccination and low rates of infection in a local community can also make getting a booster of an mRNA vaccine unnecessary.

“Somebody who has Johnson & Johnson who lives in San Francisco — where there’s extremely low covid prevalence and there is an extremely high full-vaccination rate — they’re probably fine because they’re unlikely to get exposed to the virus anyways,” Rasmussen said.

Is it safe to boost the Johnson vaccine with an mRNA dose?

Research on mixed booster doses is underway in the United States. The National Institutes of Health recently announced the start of a clinical trial that aims to evaluate safety and immune responses to a mixed booster regimen. Atmar, one of the principal investigators, said researchers are hoping to have data on how well the approach works later this summer or in early fall. Safety information would probably be available much sooner, he said.

In the meantime, Atmar and other experts cautioned against getting a booster in the absence of data supporting that an additional dose is necessary and without a clearer understanding of the potential risks and how to best administer the shot.

“We’re not delivering covid vaccines, or any other vaccines, by the seat of our pants or by a hunch or somebody thinks it’s a good idea,” said William Schaffner, medical director of the National Foundation for Infectious Diseases, a nonprofit advocacy group. “These are very carefully crafted recommendations that look at the benefits, and they also look at the side effects.”

While side effects and adverse events related to getting the standard two doses of an mRNA vaccine or one shot of J&J are well documented, experts said similar risk assessments have not been done for boosters.

“Are you going to get many more sore arms, rashes, feeling badly, fever, not feeling up to snuff for a day or two or even longer?” said Schaffner, a professor of infectious diseases and preventive medicine at Vanderbilt University Medical Center. “We don’t know how frequently that will occur, and we don’t know anything about really any other more serious adverse reactions. I think none would be expected, I would like to make that clear, it’s just that we don’t have this experience documented.”

Timing is another factor to consider. The interval between becoming fully vaccinated and needing a booster hasn’t been determined, Atmar said. In the NIH clinical trial, participants will receive a booster dose 12 to 20 weeks after completing their initial regimen.

Yet some emerging data supports mixing adenovirus and mRNA vaccines.

In England, results from the Com-CoV clinical trial showed that giving a person an AstraZeneca vaccine, and then a Pfizer dose weeks later, is safe. Study author Matthew Snape, an associate professor of pediatrics and vaccinology at England’s University of Oxford, said that based on the study results, combining an adenovirus vaccine and an mRNA vaccine “looks like they’re going to work well together.”

The combination generated additional antibodies and immune cells when compared with two AstraZeneca doses. Whether those bonus immune fighters actually matter isn’t clear, Snape said; the trial was not large enough to provide evidence for increased protectiveness.

He cautioned against over-interpreting these results. “The reason we did this study was not to look for the best immune schedule,” Snape said. He said the motivation for the study was to see whether vaccines could be combined if that became a logistical necessity — if supply chains were disrupted, for instance.

The Com-Cov study results “are important — they’re the first data we’ve seen on mixing the two doses,” said Roy M. Gulick, chief of infectious disease at Weill Cornell Medicine in New York, who was not involved with this study. But he would be “hesitant to go from one to another.”

Rasmussen, though, said “there’s really no reason to think” getting an mRNA vaccine after a Johnson & Johnson vaccine would be unsafe. “We also routinely get different types of vaccines for the same antigen or the same pathogen,” she said. “Essentially, that’s what getting the different flu shot every year is.”

Is it even possible to access a second shot if you’ve already had a Johnson dose?

If you already received a Johnson & Johnson shot in the United States, “you may find it actually very difficult to get an mRNA vaccine,” Schaffner said. “We’ve got plenty in the refrigerator, but you’re going to be listed as fully vaccinated and without a recommendation, I think the vast majority of clinics will be hesitant.”

Rasmussen said she has heard of mixed success when people who received a Johnson & Johnson shot attempted to get an mRNA booster in the United States.

“It seems to depend, in part, on the state that you’re in and how they are tracking vaccination,” she said. “Other people have said that they’ve been turned away trying to get an mRNA vaccine.” She also emphasized she is not a physician, and individuals wondering about this should consult with their primary care doctor.

Those doctors may look to federal agencies for guidance. If they do, at the moment, they won’t find support for it. “We have no information to suggest that you need a second shot after J&J even with the delta variant,” CDC director Rochelle Walensky said Wednesday on NBC’s “Today.”

The FDA, in a statement to The Post, said: “There are no data available on the interchangeability of the [Johnson & Johnson] vaccine with other COVID-19 vaccines … The FDA would need to review any new data regarding the use of a different vaccine booster from the original vaccine an individual received to make a determination about what kind of authorization would be needed.”

Experts urged people who are worried to consider adopting additional prevention strategies, such as wearing a mask if there are high rates of transmission in your community and you’re going to be in close contact with strangers. With the delta variant circulating, Barouch said, such measures should be considered regardless of which vaccine you received.

A few scientists acknowledged there may be a temptation to finagle booster shots.

“Necessity breeds inventiveness, and we have a lot of clever people out there … There will be some people who figure out a way to game the system, or they will find a permissive clinic,” Schaffner said. “Be aware that you’re sailing in uncharted waters.”