Two-dose vaccines are typically spaced months and sometimes years apart to provide long-lasting protection from disease. As children begin to get their Covid-19 shots, emerging research in adults suggests that waiting longer between doses could have similar benefits.

Studies of longer spacing between doses of mRNA vaccines, and Canada’s recent...

Two-dose vaccines are typically spaced months and sometimes years apart to provide long-lasting protection from disease. As children begin to get their Covid-19 shots, emerging research in adults suggests that waiting longer between doses could have similar benefits.

Studies of longer spacing between doses of mRNA vaccines, and Canada’s recent endorsement of a longer interval for kids, are raising questions about the optimal dose timing as shots roll out for children in the U.S. and elsewhere. But the risk calculus is tricky during a pandemic: Delaying a second dose would leave people less protected while they’re waiting, many doctors say.

The recommended interval between the first two doses in the U.S. is three weeks for Pfizer’s vaccine, including for children ages 5 to 11, for whom the vaccine was recently authorized. The recommended interval for Moderna is four weeks, but it hasn’t yet been authorized for people under 18.

Research from Canada and the U.K. has found better immune responses and fewer infections when the first two Covid-19 shots are spaced between eight and 12 weeks apart, versus three-to-four weeks.

Photo: Michael Nagle/Xinhua/Getty Images

That timing differs from most other multidose vaccines. In a pandemic with high levels of virus circulating, a pair of doses in short succession can provide more protection faster.

But spacing doses of vaccine further apart may reduce the number of infections later and result in higher antibody levels, studies indicate.

“Under the principles of vaccinology, you get a better immune response if you have longer spacing between the first two doses,” says Monica Gandhi, an infectious-disease physician at the University of California, San Francisco.

Dr. Gandhi thinks longer intervals for newly vaccinated people, including children, could eliminate the need for a booster later. She got her 11-year-old son vaccinated with his first dose of the Covid-19 vaccine on Nov. 20. She’ll delay his second shot until January, eight weeks later.

Canada recently cleared Pfizer’s vaccine for children ages 5 to 11. An advisory committee recommended an interval of at least eight weeks between doses, citing emerging evidence that a longer stretch results in more robust immune response and makes vaccines more effective. The committee also alluded to unpublished data suggesting that longer spacing may reduce the risk of very rare side effects of myocarditis and pericarditis, inflammatory heart conditions.

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The U.K. recommends that 16- and 17-year-olds who got one dose of the Pfizer vaccine wait 12 weeks or more for their second dose. The U.K doesn’t recommend a second dose for 12- to 15-year-olds unless they’re at higher risk of developing severe illness from Covid-19.

Many U.S. experts agree that the three-to-four week dose interval for mRNA Covid vaccines made sense when the shots for adults first rolled out here and think it still makes sense now for children. A year ago, cases were surging and doctors had few other tools to fight Covid. Urgent clinical trials had tested a short interval, and the public-health goal was to maximize people’s protection as quickly as possible.

That reasoning still stands, many say, especially as cases rise again in many places, a new variant emerges, winter looms and holiday gatherings approach.

With a delay in getting the second dose, “there’s the issue of what happens in the interval,” says Stanley Plotkin,

a vaccine expert and professor emeritus of pediatrics at the University of Pennsylvania. People exposed to the virus won’t be as well protected after only one dose as after two. “So it’s a trade-off,” Dr. Plotkin says.

The first Covid-19 shot generates memory B cells, which help fight infection. Over time, the stronger memory B cells survive while the weaker ones die, notes Deepta Bhattacharya, a professor of immunobiology at the University of Arizona College of Medicine in Tucson. A second shot bolsters that protection; if it happens later in the process, when the weaker cells have died, the shot will boost a stronger set of cells.

Research from Canada and the U.K. has found better immune responses and fewer infections when the first two Covid-19 doses are spaced between eight and 12 weeks apart or more, compared with three-to-four weeks.

The Canada data comes from two studies conducted in British Columbia and Quebec that were combined and published in an October preprint, which hasn’t yet been peer-reviewed. Both studies compared spacing the first two doses of Covid-19 vaccines in adults at intervals up to 16 weeks, compared with three-to-four weeks. More than 90% of the vaccinated people had received mRNA vaccines.

Both studies found mRNA vaccine effectiveness in preventing Covid-19 infection and hospitalization was about 5-to-10 percentage points higher when spaced out seven-to-eight weeks apart versus three-to-four weeks, says Danuta Skowronski, lead epidemiologist of influenza and respiratory pathogens at the British Columbia Centre for Disease Control and lead author of the study. Vaccine effectiveness was fairly stable beyond eight weeks.

“Three-to-four weeks between doses is really short for most vaccines, quite frankly,” Dr. Skowronski says. “The only reason one would rush to go with a shorter interval than usual is if the first dose doesn’t give adequate protection and we need to jack it up superquick.”

Dr. Skowronski says children, who generally have the lowest risk of severe disease from Covid-19, seem to be the ideal group for longer spacing between vaccine doses.

“You have to ask, what is the indication to go sooner?” she says. “I think expert committees need to have a debate and discussion about that.”

Write to Sumathi Reddy at sumathi.reddy@wsj.com