The FDA’s authorization followed a Thursday recommendation from the Vaccines and Related Biological Products Advisory Committee (VRBPAC), made up of virologists and other medical experts, which urged the FDA to make the Pfizer vaccine available on an emergency basis.

Here is what we know about the Pfizer vaccine, including how effective it is, how widely available it will be, who will have access to it first, how much it may cost—and whether it will end the pandemic.

How effective is the vaccine?

Compared with most vaccines, the Pfizer coronavirus vaccine is extremely effective. The FDA has affirmed Pfizer’s finding that its vaccine is 95% effective in protecting recipients from becoming infected with COVID-19. That’s much better than the flu vaccines Americans line up to get in a typical fall, which are at most 60% effective.

It was also recently confirmed by the FDA that this effectiveness includes preventing severe cases, not just mild or moderate infections.

Just as important, the vaccine shows similar effectiveness across age, gender, and racial groups, although Pfizer’s trial subjects were disproportionately (80%) white, so that data may be incomplete.

The first round of trials did not include recipients under 16 years of age, and the timeline of vaccine approval for children is currently unclear.

What are the side effects of the vaccine?

Broadly, the vaccine has minimal side effects, based on tens of thousands of trial recipients and thousands more already receiving the vaccine in the U.K. Side effects can include fatigue, aches, or flu-like symptoms lasting about a day. There were four reported cases of recipients suffering Bell’s palsy, an often temporary and non–life threatening paralysis of facial muscles, but the FDA has said those occurrences were consistent with the rate in the general population.

Could the vaccine trigger an allergic reaction?

However, two British health workers with a history of severe allergies did have serious reactions after receiving the vaccine. British health authorities are advising caution for patients with similar histories while they investigate how serious the risks might be.

How many doses of the vaccine will be available after it is approved?

Initially, very few. Pfizer and U.S. officials currently project that 40 million doses of the vaccine will reach the United States by the end of 2020. That’s enough to give the two-dose vaccine to 20 million people, or 6% of the U.S. population. Another 80 million doses, enough for 40 million Americans, will be available in 2021. But because the Trump administration declined to lock in additional supply, no doses beyond that are expected to be available until July 2021.

However, this may not be as scary as it sounds, because other vaccine candidates—particularly Moderna’s—are close behind Pfizer’s in the approval pipeline.

Who will be eligible to receive the vaccine first?

With so few doses initially available, the Centers for Disease Control recommends prioritizing medical staff and the residents of long-term-care facilities. But distribution is being handled separately by each state. The CDC has compiled each state’s vaccine game plan here.

How much will the vaccine cost?

The U.S. government has paid Pfizer and BioNTech $39 per dose for the first 100 million doses, and Americans are expected to pay very little out of pocket to be vaccinated. Medicaid, Medicare, and most forms of private insurance will be required by government mandate to cover vaccine costs for patients. Even uninsured patients should be able to get vaccinated at low or no cost: A special fund has been set up to reimburse providers for those shots.

What’s an Emergency Use Authorization?

The FDA gave the green light to the vaccine on Friday by issuing an Emergency Use Authorization, or EUA. Crucially, this is not a form of FDA approval, but a go-ahead to use the vaccine before it obtains full approval. Pfizer will have to continue the process for full approval, including conducting more trials.

What is the FDA evaluating?

The FDA authorization followed the recommendation of the advisory committee on Thursday. The panel of medical experts issued its recommendation after reviewing findings about the vaccine’s basic safety and effectiveness.

The committee on Thursday also discussed secondary implications of the emergency authorization, including questions of how to continue randomized trials after the emergency authorization is approved. The issues were not a barrier to the EUA.

What is in the vaccine?

Unlike most current vaccines, the Pfizer vaccine does not contain material from the virus it’s designed to block. Instead, it contains synthesized messenger RNA, or mRNA, a kind of genetic cheat sheet that mimics COVID-19’s biology just enough to train your body’s immune response to fight it. That mRNA is encased in a shell made of lipids, or fats, for delivery.

Why does the Pfizer vaccine require two doses?

Pfizer’s vaccines require two doses, three weeks apart. This is primarily to increase its effectiveness. The FDA has released data showing the vaccine is somewhat effective after one dose, but both doses are needed to build up full immunity.

How long is the vaccine effective?

This is still unknown because the vaccine trials were so recent. The World Health Organization hopes the vaccine is effective for at least six months. Experts speaking to New Scientist expect annual booster shots to be necessary.

Can you still transmit COVID after being vaccinated?

Trials for the Pfizer vaccine measured its safety and its effectiveness in preventing symptoms in individuals. It did not determine whether immunized people could still carry the coronavirus without showing symptoms. Mario Ostrowski, an immunology professor at the University of Toronto, told Canada’s CTV News that we will only find out if vaccinated people can be asymptomatic transmitters if infection rates remain high after a majority of the population is vaccinated. That means that masks and social distancing will remain the norm even after the vaccine has reached tens of millions of Americans.

Will the vaccine end the pandemic?

Eventually—but not immediately. Even if the Pfizer vaccine halts transmission as well as symptoms, highly infectious diseases like COVID-19 require very high levels of immunity to break the “chain of transmission” that can keep an infectious disease circulating in a population. For measles, which is also highly infectious, that threshold is 94%. It seems likely it will take most of next year to get to that level of COVID immunity in the U.S., through a combination of vaccinations and those already infected. Until that happens, the pandemic won’t really be over.

This story has been updated to reflect the FDA’s emergency authorization of the Pfizer vaccine on Friday.