As the Delta variant spreads around the world, researchers are tracking how well vaccines protect us against it and have gotten different responses.
In the UK, researchers reported in May that two doses of the Pfizer-BioNTech vaccine were as effective as 88 percent in protection against symptomatic Delta disease. In June, a study conducted in Scotland concluded that the vaccine had a 79 percent effectiveness against the variant. On Saturday, a team of researchers from Canada calculated that its effectiveness was of the 87 percent.
And on Monday, the Israeli Ministry of Health ad that the effectiveness of the Pfizer-BioNTech vaccine was 64 percent against all coronavirus infections, against the 95 percent May, before the Delta variant began its ascent toward the almost total dominance In Israel.
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–Although the range of these numbers may seem confusing, vaccine experts say it was to be expected, because it is difficult for a single study to accurately determine the effectiveness of a vaccine.
“We just have to take it as little pieces of a puzzle, and not put too much weight on any number,” said Natalie Dean, a biostatistician at Emory University.
In clinical trials, it is (relatively) easy to measure how well vaccines work. Researchers randomly assign thousands of volunteers to receive a vaccine or a placebo. If the vaccinated group has a lower risk of getting sick, scientists can be sure that the vaccine is what protected them.
But when vaccines reach the real world, it becomes much more difficult to measure. its effectiveness. Scientists can no longer control who gets a vaccine and who doesn’t. If you compare a group of vaccinated people to a group of unvaccinated people, other differences between the groups could influence their risks of getting sick.
It is possible, for example, that people who choose not to get vaccinated are more likely to experience situations where they could be exposed to the virus. On the other hand, older people may be more likely to be vaccinated, but also have a harder time defending themselves against an aggressive variant. Or an outbreak may affect a part of a country where most people are vaccinated, leaving under-vaccinated regions unharmed.
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–One way to rule out these alternative explanations is to compare each vaccinated person in a study with a counterpart who did not receive the vaccine. Often times, researchers go to great lengths to find an unvaccinated partner, looking for people who are the same age and health. They can even match people within the same neighborhood.
“It takes a lot of effort,” said Marc Lipsitch, an epidemiologist at Harvard University’s TH Chan School of Public Health.
For its new study, Israel’s Ministry of Health did not do its best to rule out other factors. “I am afraid that the current analysis from the Israeli Ministry of Health cannot be used to evaluate it safely, one way or another,” wrote Uri Shalit, a professor at the Israel Institute of Technology, on his Twitter account.
The numbers for Israel could also be different due to the people being tested. Much of the country is vaccinated. But during spikes in new local infections, the government requires that anyone, with symptoms or not, who came into contact with a person diagnosed with COVID-19 be tested. In other countries, it’s more common for people to get screened because they already feel sick. This could mean that Israel is detecting more asymptomatic cases in vaccinated people than elsewhere, reducing the reported effectiveness rate.
Fortunately, all studies so far agree that most COVID-19 vaccines are very effective in preventing people from being hospitalized and generally protect against the Delta variant. The Israel Ministry of Health estimated that the Pfizer-BioNTech vaccine is approximately 93 percent effective in preventing serious illness and hospitalizations.
“Its overall implications are consistent: Protection against serious complications remains very high,” said Naor Bar-Zeev, associate professor at the Johns Hopkins-Bloomberg School of Public Health.
Because effectiveness studies are so complicated, more work will be needed to determine how big of a threat Delta poses to vaccines. Lipsitch said studies from more countries would be needed.
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–“If there are five studies with one outcome and one investigation with another conclusion, I think you can conclude that the first five are more likely to be more correct,” Lipsitch said.
Carl Zimmer writes the column Matter. He is the author of fourteen books, including Life’s Edge: The Search For What It Means To Be Alive. @carlzimmer • Facebook
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