The Food and Drug Administration issued its first emergency use authorization of a COVID-19 vaccine Friday, providing a glimpse of the end to a pandemic that has resulted in mass death, poverty and hardship.

The history-making vaccine co-developed by BioNTech and Pfizer is the fastest ever made available in the United States, coming 11 months after a genetic sequence of the novel coronavirus was published by Chinese researchers. The FDA has taken pains to assure a wary public that speed has not come at the expense of a rigorous and transparent review process.

The announcement came in a letter late Friday from FDA Chief Scientist Denise M. Hinton to Pfizer. The agency’s independent advisers — top experts in vaccinology, immunology and infectious diseases — overwhelmingly recommended on Thursday that the Pfizer/BioNTech vaccine be authorized.

“Today’s emergency use authorization of the Pfizer-BioNTech COVID-19 Vaccine holds the promise to alter the course of this pandemic in the United States,” said Peter Marks, the director of the FDA’s Center for Biologics Evaluation and Research, in a statement.

 “Efforts to speed vaccine development have not sacrificed scientific standards or the integrity of our vaccine evaluation process,” he added. 

The vaccine is about 95 percent effective and safe for most people. The vaccine is likely to cause side effects like headache, chills and muscle pain in the days after getting a second shot, but experts say that’s a positive sign the body is beginning to generate protection.

FDA conducted its own analysis of the results of a randomized, placebo-controlled clinical trial of about 44,000 people that Pfizer and BioNTech submitted to the regulators, plus answers to follow-up questions from FDA. The vaccine requires two shots, 21 days apart.

Top administration officials like National Institute of Allergy and Infectious Diseases Director Anthony Fauci have recently estimated that a large majority, perhaps about 75 percent, of the population would need to be vaccinated to resume the pace of pre-pandemic life. But by prioritizing people most likely to catch the virus and most likely to become severely ill, they also estimate deaths could begin to drop far sooner.

The vaccine is being made temporarily available through an emergency use authorization, a power granted during public health emergencies to make medical breakthroughs available before full approval. The four criteria for an emergency use authorization are that it tackles a pathogen that can cause serious illness or death; there’s reason to believe it’s effective; the benefits outweigh the risks; and no widely available alternative exists.

However, the FDA set out a more rigorous review standard for the COVID-19 vaccines, requiring clinical trials of tens of thousands of people, setting a 50 percent bar for efficacy, and requiring manufacturing safety data. The agency made clear that having robust data on potential side effects was particularly important given the vaccine will be broadly administered to millions of healthy people.

Experts say the FDA’s requirements for the emergency use authorization of a COVID-19 vaccine more closely resembles the requirements for a full approval than an EUA for a drug like hydroxychloroquine.

Particularly contentious was the FDA’s requirement that participants be observed for a median of two months in case late side effects emerged.

That requirement reportedly rankled the White House because it delayed clinical trial results until after Election Day. Political appointees suppressed the release of FDA’s more rigorous standards, according to press reports. The information was eventually published outside of the typical channels in briefing documents to the agency’s independent vaccine advisors.

There were some key changes to typical procedures that allowed the drugmakers and the FDA to move more quickly.

The clinical trials aren’t collecting as many months of safety data as FDA typically requires, and the FDA won’t be conducting its own independent tests at manufacturing plants for safety and consistency, which also takes a few months. The FDA also sped up its review of the data. While vaccine reviews typically take about six to 10 months to crunch the numbers, the agency cut down the time to 21 days.

It will be tricky for many hospitals and state public health departments to begin to vaccinate due to funding challenges and the instability of the vaccine, which requires storage in ultra-deep freezers or containers that are continually replenished with dry ice. Many states won’t be immediately delivering Pfizer/BioNTech vaccines to long-term care facilities like nursing homes because of their sensitive temperature requirements.