Fully vaccinated people are still getting infected with Covid. Experts explain why

People who are fully vaccinated against Covid-19 are highly protected against severe infection, hospitalization and death caused by the virus. But coronavirus cases among the fully vaccinated — so-called “breakthrough” Covid cases — are still being seen among those who have had two doses.

This is happening for a number of reasons, experts note. For a start, none of the vaccines being deployed in the U.S. or Europe are 100% effective at preventing infection. In addition, new Covid strains such as the highly infectious delta variant — which is now prevalent around the world —have complicated the efficacy picture. There is also incomplete data into how long immunity from Covid lasts following vaccination.

The alarm was raised over breakthrough Covid cases when preliminary data in Israel — which had one of the fastest vaccination programs in the world — published in late July found that the Pfizer-BioNTech Covid-19 vaccine was just 40.5% effective at preventing symptomatic disease. The analysis, which was carried out as the delta variant became the country’s dominant strain, still found that having two doses of the shot provided strong protection against severe illness and hospitalization, however, the country’s Health Ministry reported.

The data also appeared to show a waning effectiveness of the Pfizer-BioNTech shot too, with the vaccine only 16% effective against symptomatic infection for those individuals who had two doses of the shot back in January. For people that had received two doses by April, the efficacy rate (against symptomatic infection) stood at 79%, however. But a study in England carried out from April to May found that, after two doses, the Pfizer-BioNTech vaccine was 88% effective against symptomatic disease caused by the delta variant.

Comparing the results is tricky, however, given differences in the nature of the vaccination programs in both countries (Israel gave all its adult population the Pfizer vaccine, for example, while in the U.K. there are several vaccines in use, with the Pfizer-BioNTech shot predominantly given to younger people) as well as differences in the study dates, Covid testing regimes and age groups.

The second factor, he added, related to “breakthrough infections in vaccinated individuals due to the more infectious delta variant” which added weight to the case for booster vaccination programs, he said. As yet, the jury is still out on booster programs with a decision yet to be made in the U.S. and U.K.

Fully vaccinated people are still getting infected with Covid. Experts explain why, CNBC, Aug 10