Last Monday, the public woke to the news that the COVID-19 vaccine developed by Pfizer and BioNTech, which has been undergoing testing since the end of July, appears to be working. That is very good news for older people, who have been hardest hit by the coronavirus epidemic, as well as for the younger population, which is bearing the brunt of the current surge in cases. And the news is very timely, as the cumulative number of cases in the U.S. is now over 11 million, with the number of new cases every day higher than ever before. But what, exactly, do we know about how effective this vaccine is likely to be?
The statistic that is cited in the news reports is that to date, the vaccine is 90 percent effective. What that means is that among the 94 people enrolled in the Pfizer/BioNTech study who were diagnosed with symptomatic infection, only 10 percent or about 9 people had received the vaccine; the other 90 percent or about 85 people had been given placebo. This is very encouraging, since the clinical trial enrolled 44,000 volunteers, half of whom received active vaccine and half of whom received placebo: it is very unlikely that such a large differential could have happened by chance. On the other hand, there’s much we don’t know.
We don’t know, for instance, whether the 90 percent effectiveness rate will hold up in all age groups. The study population does include older individuals—the plan was to try to ensure that 40 percent of those enrolled would be over age 55, though it’s unclear what percent would be in the highest risk group, those in their 80s and older. But we don’t know anything about the age or other risk factors of the 94 people who were diagnosed with coronavirus. Since older people todare being far more risk averse on average than their younger counterparts, it’s possible that none of the 94 people with infection identified so far are older adults.
We also don’t know whether the vaccine protects people against developing asymptomatic infection. From a clinical perspective, it’s more important to know whether the vaccine prevents people from developing symptoms, but from a public health perspective, we would like to know whether the vaccine keeps the virus at bay just enough so they remain asymptomatic but not enough to prevent them from transmitting the disease to others. Since asymptomatic transmission accounts for many cases today, it would be desirable to know whether the vaccine allows people to become asymptomatic carriers. We are not going to know the answer to that question as the study protocol does not call for enrollees to be tested for COVID-19 unless they develop symptoms.
Finally, we don’t know how long immunity will last, assuming the promising early results are maintained when the study is completed, which will happen once 164 subjects have been diagnosed with COVID-19 (the pre-specified endpoint of the study).
What does all this mean for everyone who is eagerly awaiting a vaccine to end this long period of isolation, anxiety, and loss? If the final data, when evaluated by the FDA, possibly by early December, do lead to approval and licensing of the vaccine, older people should be vaccinated as soon as possible—assuming the age-specific effectiveness holds up.
How will life change after you have been vaccinated? First, it should be stressed that “being vaccinated” means receiving 2 injections, 3 weeks apart. The vaccine effectiveness is being measured starting one week after receipt of the second dose, so you cannot expect protection until one month after your first shot—and you should be sure to get both shots. Second, while 90 percent effectiveness is pretty good, it’s not perfect. No vaccine is perfect, so don’t wait around for a better one. While you will face a much smaller risk of becoming sick with Covid-19 if you have been vaccinated than if you have not been, how likely you are to encounter the virus will depend on how widespread it is in the surrounding community. If, to take the extreme but unfortunately not entirely improbable case in which the rate of infection in the community goes up ten-fold, then if your risk by virtue of vaccination goes down ten-fold, the net improvement is zero. Of course, if the rate in the community goes up by a factor of ten and you haven’t been vaccinated, your risk also goes up by a factor of ten. In short, you are much better off with the vaccine than without it, but how much better off you will be will be determined by what is going on around you.
So, yes, there is good news about vaccines and yes, you should get the shots as soon as they are available, assuming the early results are confirmed and apply to older people. But don’t throw out your masks and don’t expect to go to movies and concerts or other large indoor gatherings just yet.
As I prepare to publish this blog post, news is breaking about a second vaccine made by the pharmaceutical company Moderna in partnership with NIH. More to come about these results in my next post….