The world is worried about the delta virus variant. Studies show that vaccines are effective against it.

As the delta variant continues to spread around the world, researchers are tracking how well vaccines protect against it — and are getting varying answers.

In the UK, researchers reported in May that the effectiveness of two doses of the Pfizer-BioNtech vaccine was 88 percent Prevention of symptomatic disease from Delta. A June study from Scotland concluded that the vaccine was 79 percent Effective against variants. On Saturday, a team of researchers in Canada assessed its effectiveness 87 percent.

and on Monday the Israeli Ministry of Health announced The effectiveness of the Pfizer-BioNTech vaccine was 64 percent against all coronavirus infections, which is approximately . was below 95 percent In May, before the start of the ascent of the Delta version almost total dominance in Israel.

While the range of these numbers may seem confusing, vaccine experts say this is to be expected, as it is difficult to accurately determine the effectiveness of a vaccine for a single study.

“We just have to take everything together as little pieces of a puzzle, and not put too much weight on any one number,” said Emory University biostatistician Natalie Dean.

In clinical trials, it is (relatively) easy to measure how well do vaccines work. Researchers randomly assigned thousands of volunteers to receive either a vaccine or a placebo. If the vaccinated group has a low risk of getting sick, scientists can be sure that it is the vaccine that has protected them.

But once vaccines get into the real world, it becomes much harder 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, other differences between groups may affect their risk of getting sick.

For example, it is possible that people who do not want 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 have a harder time preventing an aggressive version as well. Or an outbreak could occur in the part of the country where most people are vaccinated, causing no harm to areas with fewer vaccinations.

One way to rule out these alternative explanations is to compare each vaccinated person in a study to the counterpart who did not receive the vaccine. Researchers often go to great lengths to find a non-vaccination match, looking for people of a similar age and health. They can even match people from the same neighborhood.

“It’s a huge effort,” said Mark Lipsich, an epidemiologist at the Harvard T.H. Chan School of Health.

For its new study, Israel’s health ministry did not take such great lengths to rule out other factors. “I fear that the current Israeli MoH analysis cannot be used to safely assess it, one way or another,” said Uri Shalit, Technion – a senior lecturer at the Israel Institute of Technology, wrote on Twitter.

Israel’s numbers may also be different because of who is being tested. Vaccination is done in most parts of the country. During local bursts of new infections, the government requires testing for anyone – symptomatic or not – who may have come into contact with a person diagnosed with COVID-19. In other countries, it is more common for people to get tested because they are already feeling sick. This could mean that Israel is seeing more asymptomatic cases among vaccinated people than other places, reducing their effectiveness rates.

Fortunately, all studies to date agree that most COVID-19 vaccines are very effective in keeping people out of hospital and generally protect against the delta variant. Israel’s Ministry of Health estimated that the Pfizer-BioNTech vaccine is about 93 percent effective in preventing serious illness and hospitalization.

“Their overall effects are consistent: The protection against serious disease is very high,” said Naor Bar-Zeev, an associate professor at the Johns Hopkins Bloomberg School of Public Health.

Because effectiveness studies are so difficult, it will take more work to determine just how big of a threat delta vaccines are. Dr. Lipsich said studies from more countries would be needed.

“If there are five studies with one result and one study with another, I think one can conclude that five is probably more likely than one to be correct,” said Dr. Lipsitch said.

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