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Last Updated : Nov 21, 2020 10:01 AM IST | Source: New York Times

Two companies say their COVID-19 vaccines are 95% effective. What does that mean?

It's possible that the effectiveness of coronavirus vaccines will match their impressive efficacy in clinical trials. But if previous vaccines are any guide, effectiveness may prove somewhat lower.

New York Times
Representative Image (Image: Reuters)
Representative Image (Image: Reuters)

The front-runners in the vaccine race seem to be working far better than anyone expected. Pfizer and BioNTech announced this week that their vaccine had an efficacy rate of 95 percent. Moderna put the figure for its vaccine at 94.5 percent. In Russia, the makers of the Sputnik vaccine claimed their efficacy rate was over 90 percent.

"These are game changers," said Dr. Gregory Poland, a vaccine researcher at the Mayo Clinic. "We were all expecting 50 to 70 percent." Indeed, the Food and Drug Administration had said it would consider granting emergency approval for vaccines that showed just 50 percent efficacy.

From the headlines, you might well assume that these vaccines — which some people may receive in a matter of weeks — will protect 95 out of 100 people who get them. But that's not actually what the trials have shown. Exactly how the vaccines perform out in the real world will depend on a lot of factors we just don't have answers to yet — such as whether vaccinated people can get asymptomatic infections and how many people will get vaccinated.

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Here's what you need to know about the actual effectiveness of these vaccines.

COVID-19 Vaccine

Frequently Asked Questions

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How does a vaccine work?

A vaccine works by mimicking a natural infection. A vaccine not only induces immune response to protect people from any future COVID-19 infection, but also helps quickly build herd immunity to put an end to the pandemic. Herd immunity occurs when a sufficient percentage of a population becomes immune to a disease, making the spread of disease from person to person unlikely. The good news is that SARS-CoV-2 virus has been fairly stable, which increases the viability of a vaccine.

How many types of vaccines are there?

There are broadly four types of vaccine — one, a vaccine based on the whole virus (this could be either inactivated, or an attenuated [weakened] virus vaccine); two, a non-replicating viral vector vaccine that uses a benign virus as vector that carries the antigen of SARS-CoV; three, nucleic-acid vaccines that have genetic material like DNA and RNA of antigens like spike protein given to a person, helping human cells decode genetic material and produce the vaccine; and four, protein subunit vaccine wherein the recombinant proteins of SARS-COV-2 along with an adjuvant (booster) is given as a vaccine.

What does it take to develop a vaccine of this kind?

Vaccine development is a long, complex process. Unlike drugs that are given to people with a diseased, vaccines are given to healthy people and also vulnerable sections such as children, pregnant women and the elderly. So rigorous tests are compulsory. History says that the fastest time it took to develop a vaccine is five years, but it usually takes double or sometimes triple that time.

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What do the companies mean when they say their vaccines are 95% effective?

The fundamental logic behind today's vaccine trials was worked out by statisticians more than a century ago. Researchers vaccinate some people and give a placebo to others. They then wait for participants to get sick and look at how many of the illnesses came from each group.

In the case of Pfizer, for example, the company recruited 43,661 volunteers and waited for 170 people to come down with symptoms of COVID-19 and then get a positive test. Out of these 170, 162 had received a placebo shot, and just eight had received the real vaccine.

From these numbers, Pfizer’s researchers calculated the fraction of volunteers in each group who got sick. Both fractions were small, but the fraction of unvaccinated volunteers who got sick was much bigger than the fraction of vaccinated ones. The scientists then determined the relative difference between those two fractions. Scientists express that difference with a value they call efficacy. If there’s no difference between the vaccine and placebo groups, the efficacy is zero. If none of the sick people had been vaccinated, the efficacy is 100 percent.

A 95 percent efficacy is certainly compelling evidence that a vaccine works well. But that number doesn't tell you what your chances are of becoming sick if you get vaccinated. And on its own, it also doesn't say how well the vaccine will bring down COVID-19 across the United States.

What's the difference between efficacy and effectiveness?

Efficacy and effectiveness are related to each other, but they're not the same thing. And vaccine experts say it’s crucial not to mix them up. Efficacy is just a measurement made during a clinical trial. "Effectiveness is how well the vaccine works out in the real world," said Naor Bar-Zeev, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.

It's possible that the effectiveness of coronavirus vaccines will match their impressive efficacy in clinical trials. But if previous vaccines are any guide, effectiveness may prove somewhat lower.

The mismatch comes about because the people who join clinical trials are not a perfect reflection of the population at large. Out in the real world, people may have a host of chronic health problems that could interfere with a vaccine’s protection, for example.

The Centers for Disease Control and Prevention has a long history of following the effectiveness of vaccines after they are approved. On Thursday, the agency posted information on its website about its plans to study the effectiveness of coronavirus vaccines. It will find opportunities to compare the health of vaccinated people to others in their communities who have not received a vaccine.

What exactly are these vaccines effective at doing?

The clinical trials run by Pfizer and other companies were specifically designed to see whether vaccines protect people from getting sick from COVID-19. If volunteers developed symptoms like a fever or cough, they were then tested for the coronavirus.

But there's abundant evidence that people can get infected with the coronavirus without ever showing symptoms. And so it's possible that a number of people who got vaccinated in the clinical trials got infected, too, without ever realizing it. If those cases indeed exist, none of them are reflected in the 95 percent efficacy rate.

People who are asymptomatic can still spread the virus to others. Some studies suggest that they produce fewer viruses, making them less of a threat than infected people who go on to develop symptoms. But if people get vaccinated and then stop wearing masks and taking other safety measures, their chances of spreading the coronavirus to others could increase.

"You could get this paradoxical situation of things getting worse," said Bar-Zeev.

Will these vaccines put a dent in the epidemic?

Vaccines don't protect only the people who get them. Because they slow the spread of the virus, they can, over time, also drive down new infection rates and protect society as a whole.

Scientists call this broad form of effectiveness a vaccine's impact. The smallpox vaccine had the greatest impact of all, driving the virus into oblivion in the 1970s. But even a vaccine with extremely high efficacy in clinical trials will have a small impact if only a few people end up getting it.

"Vaccines don't save lives," said A. David Paltiel, a professor at the Yale School of Public Health. "Vaccination programs save lives"

On Thursday, Paltiel and his colleagues published a study in the journal Health Affairs in which they simulated the coming rollout of coronavirus vaccines. They modeled vaccines with efficacy rates ranging from high to low but also considered how quickly and widely a vaccine could be distributed as the pandemic continues to rage.

The results, Paltiel said, were heartbreaking. He and his colleagues found that when it comes to cutting down on infections, hospitalizations and deaths, the deployment mattered just as much as the efficacy. The study left Paltiel worried that the United States has not done enough to prepare for the massive distribution of the vaccine in the months to come.

"Time is really running out," he warned. "Infrastructure is going to contribute at least as much, if not more, than the vaccine itself to the success of the program."

 

c.2020 The New York Times Company
First Published on Nov 21, 2020 10:00 am
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