Scientist Xinhua Yan works in the lab at Moderna in Cambridge, Massachusetts on February 28, 2020.

Two coronavirus vaccine candidates now lead the pack.

Moderna, a Massachusetts-based biotech firm, announced Monday that its vaccine is 94.5% effective in preventing Covid-19. It's the second company to report positive results from the final stage of clinical trials: Last week, Pfizer and BioNTech reported that their vaccine was found to be more than 90% effective.

"Now we have two vaccines that are really quite effective. I think this is a really strong step forward to where we want to be about getting control of this outbreak," Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told NBC's Today on Monday.

Both Moderna and Pfizer's vaccines require two shots. Moderna's two doses are administered a month apart, while Pfizer's are given three weeks apart.

Many other vaccines — including the one that protects against measles, mumps, and rubella — also require back-to-back doses to be most effective. Other coronavirus vaccine candidates still in trials may require two shots as well, including AstraZeneca's; the company's phase 3 trial is testing two shots given four weeks apart. Johnson & Johnson, meanwhile, is testing both a single-dose and a two-dose vaccine in simultaneous phase 3 trials.

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

Keeping the vaccines cool

In earlier phases of their clinical trials, both Moderna and Pfizer reported that participants' virus-neutralizing antibody levels increased after a second dose. 

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

Moderna's shot, at least, doesn't require a deep-freeze. It can be stored at standard refrigerator temperatures for up to a month, the company said on Monday

The Pfizer vaccine, however, needs to be shipped and stored at minus 70 degrees Celcius — more than 100 degrees colder than standard refrigeration temperatures. Once it's moved to a standard fridge, it only keeps for five days

So getting Pfizer's shot from where it's produced into patients' arms requires dry ice, special freezers, and a storage timeline with little room for error. That poses a formidable challenge for distribution in rural areas and the developing world, particularly in places that lack reliable electricity or the funds to purchase special freezers.

It could also 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, the former director of the US National Immunization Program, previously told Business Insider.

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

We may need 'revaccination at periodic intervals'

Challenges could 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. That's not because of new, distinct strains, which is the primary reason we need a new flu shot each year. Rather, research has found that that coronavirus antibodies dissipate after weeks or months, which could mean our immunity might be similarly transient.

Our immune systems have more than just that line of defence, 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," Marm Kilpatrick, a disease ecologist, previously told Business Insider.

It's not a deal-breaker if people become susceptible to reinfection, though.

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

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

Experts won't be able to answer that question, or determine whether boosters will become part of the protocol at all, until a while after the first vaccines get rolled out.

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

According to Moderna CEO Stephane Bancel, the company plans to produce 100 million doses — enough to vaccinate 50 million people — by the end of March 2021.

Andrew Dunn contributed reporting to this story.