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Opinion: The vaccine: Efficacy, effectiveness, and impact

Last month, two pharmaceutical companies reported extremely high efficacy rates for their COVID-19 vaccines: Moderna, 94.1 percent; Pfizer/BioNTech, 95 percent. Given this great news, many people might assume that the vaccines would protect 94 or 95 percent of the people who receive them.

That is not true. The data are reported for the subjects in the sample that got the vaccine under experimental conditions. How vaccines perform in the real world depends on a lot of factors. At this point, for example, we don’t know what percentage of the people who received the vaccine may have contracted the disease but were asymptomatic.

In the real world, those people could pass the virus to many others. Moreover, the ultimate effects on the total population will likely depend on how many people choose to be vaccinated. And, I suspect that the volunteers who received either the vaccine or the placebo (false vaccine) were all in good health, and that is certainly not true of the general population.

Efficacy and effectiveness

In medical trials, efficacy and effectiveness are related, but they are not synonymous. Efficacy is the measurement that is the result of clinical trials, whereas effectiveness is how the vaccine performs in the real world. An examination of the results of previous vaccines shows that effectiveness is somewhat lower than efficacy.

This phenomenon occurs because people who volunteer for the experiment are usually in better health than many people in the general population. When the vaccine is administered to people with pre-existing conditions, chronic illness, or weak immune systems it may be ineffective or even have adverse effects.

The Pfizer trials

The way that medical trials are conducted also may have a bearing on the difference between efficacy and effectiveness. Let’s take a look at the Pfizer study.

Pfizer gathered a sample of 43,661 volunteers, some of whom received the vaccine and others got an injection of salt water (the placebo). Researchers then waited until 170 people became ill with coronavirus symptoms. Of the 170 sick people, only 8 had received the vaccine. Eight divided by 170 is 0.047, or 4.7 percent, giving an efficacy rate of 95.3 percent. That’s not exactly how the percentage is arrived at, but it’s a close approximation of more complicated mathematics, which actually produce an efficacy rate of 95 percent. However, if none of the people who received the vaccine got sick, the efficacy rate would be 100 percent; if there was no difference between the two groups, then the efficacy rate would be zero.

There is a problem in relying on the results based on those 170 people. Writing for the New York Times, Farhad Manjoo states that there is “abundant evidence that people can get infected with the coronavirus without ever showing symptoms. And so it is possible that a number of people who got vaccinated in the clinical trials got infected, too, without ever realizing it.” Those infected people would not show up in the reported efficacy rate. And, that would affect the impact of the vaccine on the general population.

The Moderna trials

Consistent with the protocols of medical testing, Moderna’s trials were carried out the same way as Pfizer’s. The Moderna sample consisted of 30,000 volunteers, some of whom got the vaccine. The rest were the control group and received a placebo. After 185 people in the study got sick, Moderna ran the data.

On Monday, the company released an analysis that showed that, of the 185 who developed symptoms of the COVID-19 disease, only 11 actually got the real vaccine. That produced an efficacy rate of 94.1 percent. However, none of the 11 vaccinated people developed severe cases. That means that the drug’s efficacy rate against the incidence of the disease that is severe enough to require hospitalization is 100 percent!

Moderna has applied to the Food and Drug Administration for emergency-use authorization (EUA). Pfizer applied for EUA last week. Only one of the sick people in its vaccinated group developed the severe form of the disease. If the drugs receive approval, vaccinations for health workers could begin before the end of the year. That campaign would probably be followed by an attempt to immunize people who are most vulnerable to the disease: essential workers — like police officers — and residents in nursing homes. Vaccinations for the general population might be available by late spring.


A vaccine has “impact” because it not only protects the person who gets vaccinated, but also slows the spread of the disease through the general population. As A. David Paltiel, Professor of Public Health at Yale, told the New York Times, “Vaccines don’t save lives. Vaccination programs save lives.”

Paltiel and his colleagues simulated vaccines with various efficacy rates and distribution models. In a research article published in Health Affairs, they concluded that the “success of a COVID-19 vaccine will depend not only on its efficacy, but at least as much on how fast and widely it can be delivered, the pandemic severity, and the public’s willingness to be immunized.”

His concern about problems with the distribution of the vaccine is obvious. “Time is running out,” according to Paltiel. “Infrastructure is going to contribute as least as much, if not more, than the vaccine itself to the success of the program.”

To you and me, the key statistic is the impact that the vaccine will have on our society. In order to eradicate COVID-19, we need to have a sufficient supply of vaccine, an efficient and timely rollout of the serum, and continued vigilance and cooperation from the public. That means continuing to wear face masks, observing social distance in public, and maintaining good hand hygiene even after we’re vaccinated. If we do that, the coronavirus will someday seem like a bad dream, just like yellow fever, smallpox, and polio.

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Jim Glynn is Professor Emeritus and the author of several textbooks in sociology. He may be contacted at



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