Pfizer COVID-19 vaccine looks highly effective, even for high-risk patients, new data indicate

If you’re weighing the pros and cons of getting the two-shot vaccine made by Pfizer with partner BioNTech, the scales just tipped even more in favor of rolling up your sleeve.

The companies previously reported that the novel vaccine is about 95% effective at preventing infection with the coronavirus. Briefing documents published Tuesday by the Food and Drug Administration show that the remarkably high level of protection is true even for those at highest risk of severe COVID-19 because of their age, weight, race, or chronic medical conditions.

But if you opt to get the shots, know you may feel lousy, especially after the second dose. A day or two of fatigue, headaches, chills, and muscle pain was common among those who got the vaccine in the pivotal clinical trial.

» READ MORE: Why vaccine side effects shouldn't keep you from rolling up your sleeve.

Here’s a deeper look at the Pfizer briefs, FDA scientists’ analysis of the data, and reactions from experts following the unprecedented development of a vaccine in less than a year:

On Thursday, the briefing documents will be reviewed by the FDA’s vaccine advisers during an all-day public meeting. Then the committee will vote on whether to recommend emergency authorization of Pfizer’s immunization. The FDA is expected to decide within days, and the first doses could be shipped soon afterward.

Emergency authorization is virtually assured. Pfizer’s vaccine is far more effective than the 50% threshold set by the agency, and experts agree that the virus cannot be stopped without effective immunizations.

Even if you are eager to be vaccinated, you might have a long wait. The federal government committed to buying 100 million doses, and Pfizer expects to deliver 25 million doses by the end of the year. Those have been earmarked for frontline health-care workers and nursing homes.

» READ MORE: Health workers, frail seniors are first in line for vaccine

Pfizer will deliver the balance of the 100 million doses -- enough for 50 million people -- early next year. But the Washington Post reported that the government passed on the opportunity to buy another 100 million, and now Pfizer says additional doses might not be available until late June, given global demand.

In the clinical trial, about 44,000 volunteers in three countries were given two doses of the vaccine, or two shots of a placebo, three weeks apart. It took only two months of following them as they lived their normal lives to see a striking effect.

The placebo group had 162 cases of COVID-19, while the vaccine group had just eight cases — meaning the shots were 95% effective at warding off the coronavirus.

The briefing documents add to this picture: The protection started to kick in within 10 days of the first dose, which reduced the chance of infection by about 52%.

After the second dose, that rose to 95% and was similar “across age groups, genders, racial and ethnic groups, and participants with medical [conditions] associated with a high risk of severe COVID-19,” the FDA scientists concluded.

John Wherry, an immunologist and director of the Penn Institute of Immunology, said that puts the Pfizer vaccine in the top tier of potency. Other vaccines that are 90% or more effective include measles, diphtheria, tetanus, and polio, he said.

Seasonal flu shots, on the other hand, have been 40% to 60% effective, meaning they reduced the chance of needing to see a doctor by that much, according to the Centers for Disease Control and Prevention.

It is not clear how long Pfizer’s vaccine protection will last, but the data contained encouraging indicators. Participants had levels of coronavirus-fighting antibodies that were comparable to or higher than levels in the blood of recovered COVID-19 patients. Pfizer is continuing to evaluate how long antibodies persist. Volunteers also had strong activation of T cells, the second wave of disease-fighters that are crucial to long-term immunity.

Both the Pfizer vaccine and the Moderna vaccine, which is also expected to get emergency authorization soon, use a decades-old 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.

This approach also has a safety advantage. Traditional vaccines that use a weakened virus to prime the immune system have been found, in extremely rare cases, to cause the infection they are designed to prevent. The mRNA vaccine cannot cause COVID-19 because a protein, by itself, cannot cause infectious disease.

» READ MORE: What about 'natural immunity' after surviving COVID-19?

As for temporary, unpleasant side effects, the vaccine group had plenty. More than half of vaccinated participants ages 16 to 55 developed fatigue and headaches, a third reported chills, and 37% had muscle pain. Fatigue was just as common in older volunteers, but rates of headache, chills, and muscle pain were somewhat lower.

Since COVID-19 can cause some of the same early symptoms, patients will need to be forewarned, Kristen Choi, a psychiatric nurse and researcher at the University of California, Los Angeles, wrote in JAMA Internal Medicine.

“Clinicians will need to be prepared to discuss with patients why they should trust the vaccine and that its adverse effects could look a lot like COVID-19,” Choi wrote as part of her first-person account of being in the Pfizer trial.

Wherry, at Penn, said many vaccines cause temporary side effects and that’s a good thing because “that means your immune response has been activated.”

Whether the Pfizer vaccine can cause rare, potentially permanent bad reactions will not be clear until participants are followed longer and the vaccine is given to millions of people. FDA scientists found no “meaningful” differences in adverse event rates between the vaccine and placebo groups during two months of follow up.

However, the FDA scientists also noted that four participants, all in the vaccine group, developed a type of facial paralysis called Bell’s palsy. That number would be expected to occur in the general population without any vaccination, so it might be just a coincidence.

“There is no clear basis upon which to conclude a causal relationship at this time, but FDA will recommend surveillance for cases of Bell’s palsy with deployment of the vaccine into larger populations,” the agency scientists wrote.

Wherry said he would “absolutely” get the vaccine, but he also struck a note of caution.

”I am telling myself continually to pump the brakes,” he said. “We need more information as quickly as possible from the real-world setting.”