The Johnson & Johnson shot comes in a distant third among COVID-19 vaccinations in the United States. Of the 150 million Americans fully vaccinated, just 8% of them got the J&J vaccine. Pfizer and Moderna divide up the rest at 52% and 40% respectively.

While 8% is a slender slice, it still translates to about 12 million people, more than the combined populations of Minnesota, Iowa and both Dakotas. A group this size shouldn't be left by federal health officials to wonder whether a booster shot is needed as new, more transmissible COVID strains emerge.

It's possible that booster shots may be needed at some point by everyone regardless of whether it was Pfizer, Moderna or J&J that went into your arm. All three vaccines are highly effective in preventing serious illness and death. But there's potential for this protection to wane over time or for new variants to evade it. Scientific studies are underway to determine the need for boosters, with some answers potentially coming later this year.

But there is urgency in providing guidance sooner for those who have gotten the J&J shot. Recent developments have led to understandable questions about this vaccine's effectiveness against new viral strains. Some prominent medical experts are advocating for J&J recipients to follow up with a second shot of either the Pfizer or Moderna vaccines. The U.S. Centers for Disease Control and Prevention (CDC), the U.S. Food and Drug Administration (FDA) and the vaccine's manufacturer have been silent. That needs to change.

Concerns are rooted in J&J's differences from the two other vaccines authorized for U.S. use. The J&J vaccine only requires one shot to be fully vaccinated; Pfizer and Moderna require two shots. The J&J shot also differs from Pfizer and Moderna in that it doesn't use mRNA technology to stimulate an immune response, relying instead on a more traditional platform.

Recent research has spotlighted how critical the second Pfizer shot is in protecting against new variants. British public health officials evaluated Pfizer and another two-shot vaccine, one made by AstraZeneca (AZ), against variants — including the Delta variant, which was first detected in India and is now spreading through Europe and causing outbreaks in Missouri.

English and Scottish teams did separate studies. They both found that the first shots of Pfizer and AstraZeneca were only 33% effective at preventing symptomatic disease against Delta. But Pfizer's effectiveness surged after the second shot, with English officials reporting 88% effectiveness and Scottish officials 83%. The AZ vaccine's performance also improved after the second but not nearly as much — to around 60% in both studies.

The research raises reasonable questions about the single-shot J&J's performance against Delta. So does AZ's less impressive second-shot performance. This vaccine is not authorized in the U.S. But it stimulates an immune response similarly to the J&J vaccine. It's logical to wonder if J&J is also less effective against variants than Pfizer or Moderna.

So far, U.S. health officials have not weighed in on J&J recipients' need for a booster and whether, if needed, it should it be one of the mRNA vaccines. But Canada's National Advisory Committee on Immunization recently took an intriguing step. AZ is authorized for use there. Canadian officials are recommending that those waiting for their second AZ shot instead get a Pfizer or Moderna jab.

Debate over J&J boosters is growing. Dr. Angela Rasmussen, a prominent virologist, recently shared that she had gotten a Pfizer vaccine to "top off" her J&J shot. Others urging federal officials to consider this include Mayo Clinic's Dr. Vincent Rajkumar and Stanford University's Dr. Michael Lin.

Without further guidance from CDC or FDA, an editorial writer found, Minnesota clinics and pharmacies are reluctant to give boosters to J&J recipients. Guidance, not further silence, is critical from the CDC and the FDA.