Monday, December 14, 2020

What do we know about the recently-approved Pfizer vaccine?

As you almost certainly know, the FDA gave emergency use authorization (EUA) late Friday for the Covid-19 vaccine produced by Pfizer and BioNTech, and initial supplies have been shipped. The trial data that persuaded the FDA Advisory Committee to vote in favor of EUA was also published online by the New England Journal of Medicine, giving me and everyone else the opportunity to go beyond the companies’ press release which led to massive hype.

The trial was large enough to be credible. 43,548 volunteers were randomized, and 43,448 received injections. 21,720 got the vaccine and 21,728 got a placebo injection. At the time of the report, 18,556 had gotten both doses of vaccine, 18,530 two doses of placebo and been followed for 2 months.

The two groups were followed for the development of symptoms consistent with Covid-19 and a positive PCR test. There was a clear separation between the two groups beginning at 14 days after the first dose, with many fewer cases in vaccinated people. The pre-defined measure was the difference between vaccine and placebo starting seven days after the second dose, and this was dramatic: 172 cases in the placebo group and 9 in the vaccinated group. This is what led to the widely reported 95% efficacy rate.

Side effects were common, but generally mild. Some 80% of younger vaccine recipients (16-55) had pain at the injection site, as did 68% of those over 55. Fewer than 10% had redness or swelling. Fatigue was seen somewhat more often after the first dose and in over half the subjects after the second. Headache was barely more frequent in vaccine recipients than in the placebo group after the first dose but was 2-3 times more common after the second. Chills and muscle pain were less common but were clearly more common after the vaccine. There were no ”major” adverse events. Two vaccine recipients died, as did four who got placebo, and none of these deaths were felt by independent experts to be vaccine related.

So, over a short period, the vaccine clearly works, and over a short period its side effects are no worse than those for widely accepted vaccines such as the shingles vaccine. What don’t we know? There are still unanswered questions.

First, does it work in children? All study participants were 16 years or older. Studies are now being done in younger adolescents, but not yet in younger children.

Is it safe for pregnant women? Pregnant women were excluded from the trial. The American College of Obstetricians and Gynecologists has offered guidance on vaccinating pregnant and lactating women. The group notes that women should not be required to undergo pregnancy tests before receiving the vaccine and say that vaccines shouldn't be withheld from pregnant women who are eligible for vaccination based on priority groups outlined by the Advisory Committee on Immunization Practices. In addition, vaccination should be offered to lactating women based on their priority group. As with all “expert opinion” that is not supported by data, you must accept this as helpful but inconclusive advice.

How long will immunity last? The study followed patients for a median of two months at the time of reporting. The vaccine used a new technology. A study I referenced in my prior post gave hope that immunity would last, but even that one only had several months’ experience.

What happens if you miss the second dose? As with all clinical trials, this one had a remarkably good follow-up: 98% of those who got dose 1 also got dose 2. In the real world, I would bet house and home that this will not be replicated, though every effort will be expended to try to get people back. It is somewhat reassuring that there was at least some evidence of protection by 14 days after dose 1, a week before the second dose. How much benefit and for how long it lasts are unknown.

Does the vaccine prevent asymptomatic or mild infections? People in the study were only tested if they developed symptoms consistent with Covid-19. We know that even now, many people can carry the virus and pass it to others without being obviously sick. It is certainly possible that vaccinated individuals could catch the virus, not get sick but pass it to others. This is important information that I hope the FDA will demand from on-going studies.

Finally, while the vaccine's safety appears to be acceptable, with mostly annoying side effects, it is too early to declare the vaccine free of serious side effects.

Am I going to get the vaccine? Absolutely, as soon as it is available for me.

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8 comments:

  1. Our daughter, Kristina, Chief of Pediatric ENT at UCSF will get vaccinated this Thursday, Dec. 17. Hurray!

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  2. Great info. Thanks for the break-down of testing.
    Dr. Hoffer.

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