Most people say they are willing—even eager—to get vaccinated against the virus that causes COVID-19. Even among the majority of people who have been vaccinated or say they want the vaccine as soon as possible, however, many people are confused about what exactly these vaccines do. Among their questions are:
1. What does it mean to say vaccines are 95% effective?
2. Why are we told to continue to wear masks, practice social distancing, and wash hands frequently even after being vaccinated?
3. How does herd—or as we prefer to call it, community—immunity factor into everything?
These are all critical questions to ask and we will try to shed some light on the answers.
That 95% Effective Claim
The first part of the first question is “95% effective against what?” and the second part is “why isn’t it 100% effective?”
We’ve pointed out in other commentaries that research studies always have to have a main outcome measure when they are initiated. This prevents researchers from looking over the data after a study is completed and cherry-picking results they like while ignoring those that are inconvenient for their hypotheses. In the case of the Pfizer/BioNTech vaccine, the main outcome measure chosen before the study started was the number of people who got sick from COVID-19 in the vaccine group compared to the number in the placebo group. In other words, to be counted as an infection case in this study, a participant had to have both symptoms of COVID-19 and a positive COVID-19 test. Of the first 170 people who developed symptomatic infection, 162 were in the placebo group. That means that 95% of the symptomatic infections were in the placebo group and that’s where the 95% effectiveness rate comes from (technically, it’s efficacy, not effectiveness, as we will explain later). The same scenario is essentially the case for the Moderna/NIH vaccine.
What neither study had as a main outcome measure was whether the vaccines are also effective against preventing asymptomatic infection and we know that people with asymptomatic COVID-19 are fully capable of transmitting the disease. That does not mean that the vaccines are incapable of preventing asymptomatic infection—our best guess is that they probably are—but we just don’t have sufficient data yet to prove that they can. Further studies are needed to confirm this and they will be done.
The answer to the second part concerns that approximately 5 percent of people in the trials who got the vaccine but still developed symptomatic infection. The most likely reason for this has to do with differences among people in their immune responses. In the Pfizer/BioNTech trial, people were evaluated for symptomatic infection seven days after the second shot; in the Moderna/NIH trial it was after 14 days. It may take some people longer to develop an immune response, so it is possible that among the very few people in the trials who developed symptomatic infection after getting the real vaccine, some just hadn’t developed immunity yet.
It is also the case that a very small number of people will never develop immunity to the virus even after being vaccinated. Antibodies are made by cells in the body called B lymphocytes or B cells. The kind of antibodies any B cell can produce depends in part on the specific genes for antibody production present in that B cell. It is possible that a very few people have B cells that just won’t respond to vaccines for COVID-19 because of genetic factors.
It is also important to remember that clinical trials are conducted under ideal circumstances. People who are taking medications or who have underlying illnesses that might interfere with the way a drug works can be excluded from participating in a clinical trial but not necessarily from receiving the drug once it is approved and available to the general public. For that reason, what clinical trials tell us is called efficacy, whereas what we learn about how well a drug or vaccine works in real life is called effectiveness. It is possible that the vaccines will be slightly less effective than was shown in the clinical trials.
So right now we know that about 95% of people who receive one of the two available vaccines will be protected from getting sick with COVID-19. That’s extremely good news, but like everything else in science, not perfect.
You Still Have To Wear a Face Mask and Socially Distance
Once vaccinated, why would someone still have to wear a face mask, practice social distancing, and wash their hands frequently? We’ve actually already answered that question: we don’t know if vaccinated people are still capable of being asymptomatic carriers of the illness and asymptomatic carriers can spread it.
Estimates vary, but about 20% of people with COVID-19 are asymptomatic. Although asymptomatic carriers may be less efficient at spreading the virus than symptomatic carriers, they still do spread it. A study published last month found that more than half of new cases of COVID-19 are acquired from people who are asymptomatic or pre-symptomatic.
So, a vaccinated person might still acquire completely asymptomatic infection and be capable of spreading it to others. Until we have more data on this issue, to prevent harming others even vaccinated people need to continue to wear masks, socially distance, and wash their hands frequently. Those vaccinated people are protected against getting sick with COVID-19 themselves, but we just don’t know for sure yet whether they are protected from making others sick. That absolutely does not mean the vaccines don’t work: they work to prevent symptomatic illness and it is possible–but not yet known–that they prevent transmission.
That leaves the tricky third question—will there ever be a time when we get this coronavirus under control? That depends on whether we are able to establish community immunity.
Why do we prefer the term “community immunity” to “herd immunity?” First, because people don’t like to think of themselves as being part of a herd, we suppose. But more important, we want to emphasize the fact that we need to think in terms of communities rather than whole, national populations. We know for example that when 95% of people are vaccinated against measles, the measles virus has no hosts and can’t infect anyone. On a U.S. national level, we are close to that 95% immunization rate. We also know, however, that when an individual community slips below that 95% immunization rate, outbreaks of measles occur.
Community immunity, then, is the percentage of people in any community who need to be vaccinated in order to make it very unlikely that the virus can spread through that community. How many people need to be vaccinated against COVID-19 to establish community immunity? The CDC answer is that we don’t know yet. A number of scientists have tried to use computer models to estimate the number of people who would need to be vaccinated in order to establish community immunity and the percentage we have come across most often is about 70%. One example of this comes from a paper that was posted but not yet peer reviewed in which a group of scientists wrote “Although estimates vary, simple calculations suggest that herd immunity to SARS-CoV-2 [the virus that causes COVID-19] requires 60-70% of the population to be immune.”
That 70% figure may at first seem reassuring because it is right about at the threshold of the number of people who say in recent surveys that they will agree to be vaccinated. But it is not, unfortunately, much more than a best guess at this point. The major reason for the uncertainty is that the estimate of the percentage of people who need to be vaccinated depends in large part on a metric called R0 (pronounced “R naught”). That is the number of people an infected person can in turn infect. The higher that number, the greater the number of people that will need to be immune in order to establish community immunity.
Right now, the thing we know best about R0 for COVID-19 is that it varies from community to community depending on how close people are to each other, how well they adhere to preventative measures like face masking, and a number of other variables. The estimates for R0 typically used for COVID-19 in the computer models are around 2-3, but it is probably higher in a place like New York City or other urban areas where more people are crowded into smaller spaces. If R0 is greater than zero, more people would need to be vaccinated in order to establish community immunity.
What we can say right now is that it is critically important for everyone who is medically eligible to have a COVID-19 vaccine. It will protect most of those vaccinated from getting sick and ultimately when enough people are vaccinated—perhaps somewhere around 70% of any given community—the virus could come under control and we might just be able to cast aside face masks and get closer than six feet to each other.
This seems like a lot to ask. Vaccine roll-out has been slow and there are still a substantial number of people who are hesitant to be vaccinated. Some people insist that because they are healthy and young they wouldn’t get very sick even if they did get infected, so why have the vaccine. The answer to that is based, of course, in the desire to protect others. Healthy, young infected people need to get vaccinated if we are to have any chance at community immunity and the end of the pandemic. Let’s do everything we can to spread that message.