The vaccine as a fire hose


The vaccine news continues to seem very encouraging. Britain started its mass vaccination effort today, and the U.S. isn’t far behind.


But there is still one dark cloud hanging over the vaccines that many people don’t yet understand.


The vaccines will be much less effective at preventing death and illness in 2021 if they are introduced into a population where the coronavirus is raging — as is now the case in the U.S. That’s the central argument of a new paper in the journal Health Affairs. (One of the authors is Dr. Rochelle Walensky of Massachusetts General Hospital, whom President-elect Joe Biden has chosen to run the Centers for Disease Control and Prevention.)


An analogy may be helpful here. A vaccine is like a fire hose. A vaccine that’s 95 percent effective, as Moderna’s and Pfizer’s versions appear to be, is a powerful fire hose.

But the size of a fire is still a bigger determinant of how much destruction occurs.


I asked the authors of the Health Affairs study to put their findings into terms that we nonscientists could understand, and they were kind enough to do so. The estimates are fairly stunning:


  • At the current level of infection in the U.S. (about 200,000 confirmed new infections per day), a vaccine that is 95 percent effective — distributed at the expected pace — would still leave a terrible toll in the six months after it was introduced. Almost 10 million or so Americans would contract the virus, and more than 160,000 would die.

  • This is far worse than the toll in an alternate universe in which the vaccine was only 50 percent effective but the U.S. had reduced the infection rate to its level in early September (about 35,000 new daily cases). In that scenario, the death toll in the next six months would be kept to about 60,000.

It’s worth pausing for a moment on this comparison, because it’s deeply counterintuitive. If the U.S. had maintained its infection rate from September and Moderna and Pfizer had announced this fall that their vaccines were only 50 percent effective, a lot of people would have freaked out.


But the reality we have is actually worse.

How could this be? No vaccine can eliminate a pandemic immediately, just as no fire hose can put out a forest fire. While the vaccine is being distributed, the virus continues to do damage.

“Bluntly stated, we’ll get out of this pandemic faster if we give the vaccine less work to do."

A. David Paltiel, one of the Health Affairs authors and a professor at the Yale School of Public Health, told me.


There is one positive way to look at this: Measures that reduce the virus’s spread — like mask-wearing, social distancing and rapid-result testing — can still have profound consequences. They can save more than 100,000 lives in coming months.

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