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Why it’s so hard to know how effective vaccines are against the delta variant

Feeling confused about why newer data is showing the Pfizer vaccine is much less effective against more highly transmissible variants of COVID-19? There are a few reasons why.

Why it’s so hard to know how effective vaccines are against the delta variant
[Source Photo: Thirdman/Pexels]

Last week, a tweet from Israel’s Ministry of Health sent the world into a tizzy. It said that in Israel, the Pfizer vaccine was only 64% percent effective against symptomatic COVID-19 infection, down from 97% in May. Now, Pfizer is suggesting that a third shot—a booster—is needed, though experts disagree.

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The confusion stems from conflicting data on exactly how effective COVID-19 vaccines are against the latest and most troublesome SARS-CoV-2 variant, delta (B.1.617.2). The effectiveness rate is derived by analyzing the total number of breakthrough infections, or the number of people who are vaccinated but contract COVID-19 anyway. The reason for the diverging figures may be that it’s difficult to calculate vaccine effectiveness in an ever-changing environment, where a new variant is spreading while vaccination rates slowly tick upward. The other reason may be that not all the data is in yet.

The delta variant is extremely transmissible, 60% more than the original COVID-19 virus, and is running rampant globally. Recent data from the Canadian Immunization Research Network shows that at full dose, the Pfizer vaccine is 87% protective against symptomatic infection by the delta variant. That’s consistent with the National Health Service, which last month reported the Pfizer vaccine was 88% protective against symptomatic infection by the delta variant. However, Scotland has released data showing the Pfizer vaccine was only 79% effective against infection from the delta variant. The data from Israel now suggests that effectiveness could be even lower.

These numbers can feel especially low compared with the vaccine efficacy numbers shown in the original clinical trials. Moderna reported its vaccines had 94.1% efficacy against COVID-19 illness. Pfizer reported 95% efficacy. The new “effectiveness” numbers refer to how well a vaccine performs in the real world and are likely to be lower than inside controlled experiments.

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The Israeli government and Pfizer told the Financial Times that its most recent analysis is based on “preliminary and highly localized infection numbers and had other methodological weaknesses.” The Israeli Ministry of Health also notes that the Pfizer vaccine is still 93% effective against serious COVID-19 illness and hospitalization. Dr. Shane Crotty, a virologist on the COVID-19 task force at La Jolla Institute of Immunology, cautions that Israel’s Ministry of Health has not yet published the data behind its most recent announcement. “When they’ve released data [in the past] it’s generally been very much in alignment with other data in the world,” he says.  

Another reason for the variance in effectiveness may have to do with the evolving nature of new variants and local vaccination rates. Each of the countries presenting data has slightly different levels of vaccination (U.K. 52%, Israel 57%, Canada 46%) and the number of people getting vaccinated is changing as the data is being collected. For example, at the start of the Scotland study in April, only 7.6% of its population was fully vaccinated. By June, that figure jumped to 39.4%. It can be difficult to assess effectiveness in that changing environment.

Crotty says that epidemiologists don’t take into account the level of vaccination in a community when calculating how effective a vaccine is, though it may have an effect. “Intuitively there should be some impact,” he says. “The protection provided by any vaccine relates to how many exposures you have to the virus. If you keep getting exposed to the virus over and over, there is more likely to be a breakthrough infection. So, if no one around you is vaccinated—more exposures,” he says. Crotty is eager to add that overall the data shows the vaccines are very effective.

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This week the World Health Organization said that the majority of vaccinated people who contract COVID-19 don’t experience symptoms. Meanwhile, in the U.S., White House COVID-19 response coordinator Jeff Zients has said that virtually all COVID-19 hospitalizations and deaths now are happening among unvaccinated individuals. “And to be clear, there will likely continue to be an increase in cases among unvaccinated Americans and in communities with low vaccination rates, particularly given the spread of the more transmissible delta variant,” he said.

Still, Pfizer is looking ahead to producing a booster vaccine specifically targeting delta. The company says it has seen “encouraging” data in its third-dose trial. But government officials are more cautious. Last week, the Centers for Disease Control and Prevention and the Food and Drug Administration issued a joint statement telling vaccinated Americans they do not need a third dose. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, also said in several interviews that there’s not enough data to recommend a booster shot at this time. So far vaccines appear to be very effective, he said. Health experts agree.

“At some point in the future we may need a third shot,” Megan Ranney, an emergency physician at Rhode Island Hospital and associate professor at Brown University, told a local Rhode Island news station. But it may specifically be for people who are immunocompromised or for older Americans with multiple health issues, she said. Instead, she says we should focus on getting more people vaccinated, as the delta variant is having a profound effect on unvaccinated people, especially young people. “Until more of us show up and get this full immunization, I am worried,” she says.

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About the author

Ruth Reader is a writer for Fast Company. She covers the intersection of health and technology.

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