The Delta variant of the coronavirus may be substantially better than past strains at infecting fully vaccinated people, officials in Israel warned Monday. However, their conclusions have yet to be vetted by outside scientists, and vaccinated people still remain very well protected against more serious illness and death from the variant.
According to the Israel Ministry of Health, their analysis of recent case data (collected from June 6 to early July) suggests that the Pfizer/BioNTech vaccine is only around 64% effective at preventing any infection from Delta in the fully vaccinated, meaning both symptomatic and asymptomatic cases—a sharp contrast from the 94% effectiveness from infection found with earlier versions of the virus. The vaccine’s effectiveness against serious illness and hospitalization also appeared to decline, but only slightly, the health ministry added. They estimated the Pfizer vaccine was 94% effective at preventing serious illness from Delta, compared to 98% previously.
Delta is more transmissible than past strains of the coronavirus, including earlier variants that were themselves more transmissible than the strains that kickstarted the pandemic early last year. These findings are not the first to indicate that Delta is also more of a challenge for an immune system trained to recognize the wild-type strains of the virus that circulated throughout the world last year (and that all currently available vaccines are based on). Lab data has indicated, for instance, that the immune systems of vaccinated people produce fewer antibodies capable of effectively neutralizing Delta than they do against the wild type, and it’s these antibodies that play a large role in preventing infection from taking place at all.
But neither the data nor the analysis used to determine these estimates from Israel has been made available to the public. At least some experts within the country have questioned whether the Ministry of Health’s model may be underselling the Pfizer vaccine’s robustness in the face of Delta, according to Israeli outlet Haaretz.
The UK’s analysis of case data, which is regularly updated and released by Public Health England, has suggested that the full two doses of the Pfizer/BioNTech or the Oxford/AstraZeneca vaccines continue to provide similar protection against symptomatic illness from Delta as they did against past strains (around 88% for Pfizer). Over the weekend, researchers with the Canadian Immunization Research Network released their own preliminary research looking at the potential effects of Delta on vaccine efficacy, based on data from nearly half a million residents in the country. Like the UK, they found that full inoculation with the Pfizer vaccine was 88% effective at preventing symptomatic infection.
It’s important to keep in mind that all these vaccines were originally tested in clinical trials for their ability to prevent illness from the coronavirus, not infections altogether. Soon after their release to the public, data started to show that the mRNA vaccines made by Pfizer and Moderna were also highly effective at preventing infection, which in turn reduced the risk of transmission. If it now turns out that Delta is more likely to cause silent infections, that would be disappointing, but it wouldn’t make these vaccines failures. So far, the overall data suggests that these vaccines (including Johnson & Johnson’s one-shot version) do remain about as effective at preventing the worst effects from Delta as they did against past strains.
That said, many experts remain worried that the emergence of Delta will bring new waves of illness and death, even to highly vaccinated countries like the U.S., UK, and Israel. While most of this harm will impact the unvaccinated who haven’t been exposed to the coronavirus, it’s still crucial to clarify the exact risk fully vaccinated and previously infected people may continue to face from covid-19. And unless the pandemic is controlled everywhere, through vaccination and other public health measures, there’s always the possibility that the virus will mutate significantly enough to evade our existing immunity.