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. (John Farrell/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.