The front runners in the vaccine race appear to be performing far better than expected: Pfizer and BioNTech announced their vaccine this week had an effectiveness rate of 95 percent. Moderna came up with the number for his vaccine 94.5 percent. In Russia, the manufacturers of the Sputnik vaccine claimed its effectiveness was over 90 percent.
“These are game changers,” said Dr. Gregory Poland, a vaccine researcher at the Mayo Clinic. “We all expected 50 to 70 percent.” Indeed, the Food and Drug Administration announced that it would consider issuing an emergency license for vaccines that are only 50 percent effective.
The headlines suggest that these vaccines – which some people may receive within a few weeks – protect 95 out of 100 people who receive them. But the experiments have not shown that. Exactly how vaccines work in the real world depends on many factors that we don’t yet have answers to – such as whether people who are vaccinated can get asymptomatic infections and how many people will be vaccinated.
Here’s what you need to know about the real effectiveness of these vaccines.
What do companies mean when they say their vaccines are 95 percent effective?
The basic logic behind vaccine trials today was worked out by statisticians over a century ago. Researchers vaccinate some people and give others a placebo. They then wait for the participants to get sick and see how many of the diseases came from each group.
In the case of Pfizer, for example, the company recruited 43,661 volunteers and waited for 170 people to have symptoms of Covid-19 and then get a positive test. Of those 170, 162 had received a placebo shot and only eight had received the real vaccine.
From these numbers, the Pfizer researchers calculated the proportion of volunteers in each group who got sick. Both factions were small, but the proportion of unvaccinated volunteers who got sick was much greater than the proportion of vaccinated volunteers. The scientists then determined the relative difference between these two fractions. Scientists express this difference with a value they call effectiveness. If there is no difference between the vaccine and placebo groups, the effectiveness is zero. If none of the sick has been vaccinated, the effectiveness is 100 percent.
An effectiveness of 95 percent is certainly convincing evidence that a vaccine is working well. However, this number doesn’t tell you what your chances of getting sick are if you get vaccinated. And it alone does not say how well the vaccine will reduce Covid-19 in the US.
What is the difference between effectiveness and effectiveness?
Effectiveness and effectiveness are related, but they are not the same thing. And vaccine experts say it is crucial not to confuse them. Efficacy is just a measurement taken during a clinical study. “Efficacy is how well the vaccine works in the real world,” said Naor Bar-Zeev, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.
It is possible that the effectiveness of coronavirus vaccines matches their impressive effectiveness in clinical trials. However, when previous vaccines are used as a guide, the effectiveness may prove to be slightly inferior.
The mismatch arises because the people taking part in clinical trials do not perfectly reflect the entire population. In the real world, people can have a wide variety of chronic health problems that can affect the protection of a vaccine, for example.
The Centers for Disease Control and Prevention has long followed the effectiveness of vaccines after they are approved. On Thursday the agency sent Information on his website about his plans to study the effectiveness of coronavirus vaccines. It will find ways to compare the health of people who have been vaccinated with those in their communities who have not received the vaccine.
What exactly can these vaccines do?
The clinical trials conducted by Pfizer and other companies are specifically designed to determine if vaccines protect people from disease caused by Covid-19. If volunteers developed symptoms such as a fever or cough, they were tested for the coronavirus.
However, there is ample evidence to suggest that people can become infected with the coronavirus without ever showing symptoms. And so it is possible that a number of people who were vaccinated in the clinical trials also became infected without ever realizing it. If these cases are indeed there, none of them are reflected in the 95 percent effectiveness rate.
People who are asymptomatic can still pass the virus on to others. Some studies suggest they produce fewer viruses, making them less of a threat than infected people who continue to develop symptoms. However, if people are vaccinated and then stop wearing masks or take other safety measures, their chances of spreading the coronavirus to others can increase.
“You might get into this paradoxical situation where things are getting worse,” said Dr. Bar Zeev.
Will these vaccines contain the epidemic?
Vaccines don’t just protect the people who get them. Because they slow the spread of the virus, they can also lower new infection rates over time and protect society as a whole.
Scientists call this broad form of effectiveness the effects of a vaccine. The smallpox vaccine had the greatest impact of all, forgetting the virus in the 1970s. But even a vaccine that is extremely effective in clinical trials will have little effect if few people receive it.
“Vaccines don’t save lives,” said A. David Paltiel, professor at the Yale School of Public Health. “Immunization programs save lives.”
On Thursday, Dr. Paltiel and his colleagues a study in Health Affairs magazine simulating the impending introduction of coronavirus vaccines. They modeled vaccines with rates of effectiveness ranging from high to low, but also took into account how quickly and widely a vaccine could be distributed if the pandemic continued to rage.
The results, said Dr. Paltiel, were heartbreaking. He and his colleagues found that use was as important as effectiveness in reducing infections, hospital stays, and deaths. The study made Dr. Paltiel worries that the United States has not done enough to prepare for the vaccine’s massive spread in the coming months.
“Time is really running out,” he warned. “The infrastructure will contribute at least as much, if not more, to the success of the program than the vaccine itself.”