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As the delta variant takes over the world, researchers are monitoring how well vaccines protect against it, and are getting different answers.
In Britain, researchers reported in May that two doses of the Pfizer-BioNTech vaccine were effective. 88 percent Protection against delta-induced symptomatic disease. A study from Scotland in June concluded that there was a vaccine. 79 percent effective against the variant. On Saturday, a team of researchers in Canada pinned its activity as follows: 87 percent.
And on Monday, the Israeli Ministry of Health announced The effectiveness of the Pfizer-BioNTech vaccine against all coronavirus infections is approximately 64 percent. 95 percent In May, before the Delta variant starts climbing almost total domination in Israel.
While the range of these numbers may seem confusing, vaccine experts say this is to be expected as it is difficult for a single study to accurately determine the effectiveness of a vaccine.
“We need to put everything together as little pieces of a puzzle and not put too much weight on any one number,” said Natalie Dean, a biostatistics expert at Emory University.
In clinical studies, it is (relatively) easy to measure how well do vaccines work. Researchers randomly assigned thousands of volunteers to receive either the vaccine or a placebo. If the vaccinated group has a lower risk of getting sick, scientists can be sure it’s the vaccine that protects them.
But once vaccines reach the real world, it becomes much more difficult to measure. their effectiveness. Scientists can no longer control who gets the vaccine and who doesn’t. If they compare a group of vaccinated people with a group of unvaccinated people, there may be other differences between the two groups that affect their risk of getting sick.
For example, people who choose not to be vaccinated may be more likely to put themselves in situations where they may be exposed to the virus. On the other hand, older people may be more likely to be vaccinated, but also have a harder time fending off an aggressive variant. Or an epidemic could hit a part of a country where most people are vaccinated, leaving poorly vaccinated areas unharmed.
One way to rule out these alternative explanations is to compare each vaccinated person in a study with a colleague who did not receive the vaccine. Researchers often go to great lengths to find an unvaccinated match and look for people of similar age and health. They can even match up with people in the same neighborhood.
“It takes a lot of effort,” said March Lipsitch, an epidemiologist at the Harvard TH Chan School of Health.
The Israeli Ministry of Health didn’t go that far to rule out other factors for its new study. “I’m afraid the current Israeli Ministry of Health analysis cannot be used to safely assess it in one way or another,” said Uri Shalit, a senior lecturer at the Technion, Israel Institute of Technology. Wrote from Twitter.
Israel’s numbers may also differ due to the people tested. Most of the country is vaccinated. During local outbreaks of new infections, the government is mandating testing for anyone who has come into contact with someone diagnosed with Covid-19 – whether they have symptoms or not. In other countries, it’s more common for people to get tested because they already feel sick. This could mean that Israel is detecting more asymptomatic cases in vaccinated people than elsewhere, reducing reported efficacy rates.
Fortunately, all studies so far agree that most Covid-19 vaccines are very effective at keeping people out of hospital and generally protect against the Delta variant. The Israeli Ministry of Health estimates that the Pfizer-BioNTech vaccine is approximately 93 percent effective in preventing serious illness and hospitalizations.
“The general implications are consistent: protection against severe disease is very high,” said Naor Bar-Zeev, associate professor at the Johns Hopkins Bloomberg School of Public Health.
Because efficacy studies are so difficult, determining how much of a threat Delta poses to vaccines will require more work. Dr. Studies from more countries will be needed, Lipsitch said.
Dr. “If there are five studies with one result and one study with the other, I think it can be concluded that five is probably more accurate than one,” Lipsitch said.
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