As the nation neared 300,000 pandemic deaths, a government advisory group on Thursday recommended emergency authorization of the Pfizer-BioNTech COVID-19 vaccine for use in people ages 16 and up, a major step toward arresting the out-of-control spread of the coronavirus.

The Food and Drug Administration is expected to grant emergency approval this weekend, and the first doses, earmarked for frontline health-care workers and nursing homes, will immediately begin shipping. Next week, the same advisory panel that acted Thursday will consider whether to recommend emergency use of Moderna’s vaccine.

Pfizer’s two-shot immunization far exceeded hopes in clinical trials, demonstrating 95% effectiveness at preventing the infection that has so far claimed at least 288,000 lives in the United States. But the vaccine was developed at breakneck speed, so the safety profile, while good, is based on a scant two-month track record.

The advisers’ vote, taken after an all-day virtual meeting, was not unanimous. Seventeen members, including Paul Offit, a pediatrician and vaccine researcher at Children’s Hospital of Philadelphia, voted yes. Four members voted no, and one abstained.

In recent days, the United Kingdom, Canada, and Bahrain have given emergency approval for Pfizer’s vaccine. The U.K. has since advised people with a history of life-threatening allergic reactions not to get the shots because two health-care workers in the first round of vaccinations suffered such reactions. Both have recovered.

No serious allergic reactions were seen in Pfizer’s trials, but the company and the FDA said vaccine labeling will warn against use by people with a history of sensitivity to a chemical component of the vaccine.

Offit was not satisfied by that plan. “No one is going to look at that long chemical name on the label and say, ‘I’m allergic to that,’” he said. He also worried that millions of Americans who carry EpiPens -- devices that dispense epinephrine for serious allergic reactions to insect stings, foods, and other allergens -- may avoid vaccination because both British workers carried EpiPens.

Offit suggested that ongoing trials include people with common allergies such as eggs and peanuts, but follow them closely.

Both Pfizer and FDA officials said ongoing trials will seek to answer unknowns, including whether the vaccine can safely be used in children, pregnant women, and people with impaired immune systems. Existing and new mechanisms -- including a phone app -- will enable vaccine providers, the company, and consumers to watch for rare, serious side effects that may potentially be linked to the vaccine.

Several panel members, including pediatrician Archana Chatterjee of Chicago Medical School, wanted to hold off recommending the new vaccine for 16- and 17-year-olds because relatively few were enrolled in the trials. Chatterjee was among the “no” votes.

Eric Rubin, chair of immunology and infectious diseases at Harvard Medical School, countered that the vaccine, if authorized, would be used in a group not represented in the trial at all: nursing home residents. “I would hate to take that [adolescent] group out,” he said. “Some of these kids are working in grocery stores.”

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Offit added: “We know we don’t have a serious safety problem. Sixteen- and 17-year-olds can get this infection. We’ve seen kids in our hospital who have cardiac anomalies. We have clear evidence of benefit and on the other side, all we have on the other side is theoretical risk.”

In testing, about 44,000 volunteers in six countries, including the United States, got two shots of vaccine or a placebo, three weeks apart. The placebo group had 162 cases of COVID-19, while the vaccine group had just eight cases, which translated to a 95% reduced risk of coronavirus infection. The high effectiveness was true even in people at high risk of severe COVID-19 because of their age, weight, race, or chronic medical conditions.

Temporary, unpleasant side effects were common. More than half of vaccinated participants ages 16 to 55 developed fatigue and headaches, a third reported chills, and 37% had muscle pain, all of short duration. Fatigue was just as common in older volunteers, but rates of headache, chills, and muscle pain were somewhat lower. Many vaccines trigger such transient misery as the immune system is activated.

Both Pfizer’s and Moderna’s vaccines use a technology that has never before led to an approved product. The approach uses a tiny bit of genetic material, called mRNA, to teach human cells to make the “spike” protein that the virus uses to break into cells. This primes the immune system to attack in case a real infection occurs.

Distributing, storing, and handling the vaccine pose huge logistical challenges because the vaccine must be kept in ultra-cold freezers until a few days before use, when it can be kept in regular freezers. After thawing, the vaccine cannot be refrozen.

» READ MORE: A COVID-19 vaccine may be near, but distribution faces major hurdles

Initially, supplies will be very limited. The federal government committed to buying 100 million doses -- enough for 50 million people -- and Pfizer expects to deliver 25 million by the end of the year. But almost 18 million people, requiring 36 million doses, are in the top-priority groups -- frontline health workers and residents of nursing facilities, according to a new Kaiser Family Foundation estimate.