Pfizer's coronavirus vaccine requires 2 shots given 3 weeks apart, which could make distribution more complicated

• Pfizer's coronavirus vaccine candidate is more than 90% effective in preventing COVID-19, the company announced Monday.
• The vaccine involves two shots administered three weeks apart.
• People may later need additional booster shots too, experts say.

Pfizer's coronavirus vaccine candidate is now leading the pack, with emergency authorization expected from the Food and Drug Administration within weeks.

On Monday, the pharma giant and its German partner BioNTech announced that their vaccine was more than 90% effective, based on 94 cases of the disease observed in an interim study.

Pfizer's vaccine requires two doses to achieve that high effectiveness, though. The two shots are administered three weeks apart. In July, Pfizer said its researchers observed the highest level of virus-neutralizing antibodies one week after the participants' second dose.

Many other vaccines also require back-to-back doses to be most effective, including the one that protects against measles, mumps, and rubella. Other coronavirus vaccine candidates still in trials also involve double shots: Participants in Moderna's clinical trial get two shots four weeks apart, and AstraZeneca's ongoing trial is testing outcomes from both a single vaccine dose and two shots given one month apart.

But a two-dose vaccine comes with supply-chain challenges and the possibility that not everyone will return to a doctor's office for the critical second dose.

Double trouble

A double-dose vaccine requires twice as many vials, syringes, refrigerators, and clinic visits at a time when such resources are already limited.

The Pfizer vaccine also needs to be shipped and stored at minus 94 degrees Fahrenheit (minus 70 degrees Celsius). That poses a formidable challenge for distribution in the developing world, particularly in places that lack widespread electricity or healthcare facilities.

Plus, it could be difficult to get 100% of vaccine recipients to return for a follow-up shot.

"The more complicated the schedule, the more difficult it is to get people to come in," Walt Orenstein, a vaccinologist and former director of the US National Immunization Program, previously told Business Insider.

For example, research shows that less than one-third of young women who got the first shot of human papillomavirus vaccine (HPV) — which primarily targets viruses than cause cervical cancer — return for the remaining two doses to complete the series.

We'll likely need booster shots

Such challenges could further snowball if it turns out that people need to get revaccinated regularly.

Scientists haven't been able to study the new coronavirus long enough to determine how long immunity lasts, but some evidence suggests people could get reinfected. Research has found that that coronavirus antibodies dissipate after a period of weeks or months, which could mean our immunity — whether generated in response to an infection or a vaccine — may be similarly transient. Our immune systems have more than just that line of defense, though, so many questions remain about immunity to the virus.

"If immunity does turn out to be fleeting, we'll need a plan of a vaccination plus a booster, or revaccination at periodic intervals," disease ecologist Marm Kilpatrick previously told Business Insider.

Still, it's not a deal breaker if people become susceptible to reinfection sometime after the initial shot.

"This happens for a lot of vaccines," Florian Krammer, a vaccinologist at the Icahn School of Medicine at Mount Sinai, previously told Busines Insider. "It's not a problem. You can get revaccinated."

That's what booster shots are for. Your tetanus shot, for example, requires a booster every decade. The question is how frequently follow-up coronavirus vaccine shots may be needed.

Experts won't be able to determine whether boosters will become part of the protocol until vaccines get rolled out. Pfizer has said that the bulk of its doses — up to 1.3 billion — will be ready in 2021.

"Once we start seeing vaccine failures increasing, then we can consider booster doses. But we don't know at this stage whether that will be necessary," Orenstein said.

If it turns out that people need to be revaccinated regularly, that decreases the likelihood that everyone will get the shots they need to stay protected.

Andrew Dunn contributed reporting to this story.