Julian Montague

Editor’s Note: Every Wednesday, James Hamblin takes questions from readers about health-related curiosities, concerns, and obsessions. Have one? Email him at paging.dr.hamblin@theatlantic.com.


Dear Dr. Hamblin,

I heard an immunologist on the radio today say that a coronavirus vaccine could be only 50 percent effective, in which case we’d still have to “live” with the virus even after it arrives. With all the talk of the vaccine being the way out, this is terrifying. What if the vaccine isn’t totally effective? Will the virus really be with us forever?

Sydney Levitt

Toronto, Canada


No vaccine is perfectly effective. That isn’t bad news; it’s just a basic fact. No medicine is perfectly effective, no parachute is perfectly effective, and no person is perfectly effective at … whatever it is they do. But though vaccines are only partly effective at protecting a single person, they can still be extremely effective collectively.

Vaccine “effectiveness” takes into account lots of different factors: What percentage of vaccinated people develop antibodies? How many antibodies? How long do the antibodies last? How well do they protect the person from disease? Ultimately, you’re left with a rough average: what percentage of people who get vaccinated are protected for a meaningful amount of time.

The most successful vaccines that we have, such as those against measles, are about 97 percent effective—meaning that almost everyone develops fully protective, long-lasting immunity. Not every vaccine is so reliable. Technically, all that a vaccine does is stimulate our immune systems. From there, it’s up to our bodies to develop and maintain immunity—without inadvertently attacking our own cells in the process. Vaccines are developed to try and thread the needle of stimulating a robust and lasting antibody immune response while not making anyone too sick. As we’ve seen with the flu vaccine, which fewer than half of Americans choose to get most years, even a slight chance of a sore arm or a mild fever after a shot will deter some people from getting it—let alone a one-in-a-million chance of a more serious reaction.

The variability of our immune responses is the quintessential challenge of vaccine making. It is why safe and effective vaccines take so long to develop. Even once you have an effective formula, the process of vaccination has historically happened over decades, not months or years. The first polio inoculation studies were initiated in the 1930s, and a viable vaccine wasn’t discovered until the 1950s. Even though global eradication efforts began in the 1980s, there are still dozens of known cases of the disease every year, mostly in children. Through a century of vaccination efforts, smallpox is the only virus that has ever been totally eliminated through vaccination.

Developing a vaccine is never easy, but the coronavirus makes it uniquely challenging. The virus can affect people in many ways and to varying degrees of severity, largely because our immune systems respond very differently from person to person. When the vaccines eventually come, we should expect that some people will respond differently than others. For that reason, especially with a new vaccine that’s being produced at a fraction of previous record speeds, drugmakers and regulatory agencies could have an incentive to err on the side of under-stimulating the immune system, rather than overstimulating it and potentially causing unwanted symptoms. This would mean that fewer people would be fully protected after getting the vaccine, but might mean that more people get the vaccine. (The Food and Drug Administration doesn’t specify an exact minimum level of effectiveness in order to take a product to market.)

We don’t know how the numbers will play out yet, but we should have a preliminary sense as clinical trials wrap up near the end of the year. Each vaccine candidate—and there are dozens—could have different numbers. During the Phase 3 trials that are happening right now, people are being monitored to see whether they contract COVID-19 despite having been vaccinated. The process could have been sped up with a controversial approach known as vaccine challenge trials, in which people volunteer to be purposely exposed to the virus. But the scientific community opted not to do that for ethical reasons.

Instead, we are waiting to see whether people are infected organically. This takes time. The longer that trials go on, the more valuable the results. But even if a trial lasted five years, the results wouldn’t account for every possible long-term, real-world condition. The process can give only a window into what’s likely to happen when billions of people take the product. But do not expect a vaccine to give you 100 percent certainty of protection.

Still, if everyone in a population takes a vaccine that is, say, 70 percent effective, the effects add up quickly. The result is a population that is protected—and that more quickly achieves “herd immunity” than a population with a less effective vaccine. Occasional cases of COVID-19 might arise, but enough people will not contract the virus to prevent widespread outbreaks. When Anthony Fauci, the head of the National Institutes of Allergies and Infectious Diseases, mentioned the possibility of a vaccine being 50 percent effective, he wasn’t saying it as though the vaccine would be a failure. He was saying he would consider it a success—an intervention worth using, and better than nothing. Though, his hope would be to start with a product that’s somewhere closer to 75 percent effective.

No matter how effective the coronavirus vaccines prove to be, their overall impact will ultimately depend on how many people take them. That means how many people have access to them globally, as well as how many people consent to taking them. In a Pew Research Center poll out this week, only 21 percent of Americans surveyed said they would “definitely” take a vaccine if it were available now. This rate is half of what it was in May, and has decreased in step with the president’s unsubstantiated and impossible claims about a vaccine being widely available before the election. People will rightly require transparency and rigor from their politicians and public-health officials if a vaccine is to be widely trusted and used. The most valuable thing that any population can have in a pandemic is clear, accurate information. Without this, even a mythical, perfectly effective vaccine could fail to stop the pandemic.


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