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, or “breakthrough” Covid cases, are still being seen among those who have had two doses.
Experts note that while it’s rare for vaccinated people in the U.S. or Europe to get sick from Covid, breakthrough cases are happening for a number of reasons.
As much as people want to believe a vaccine is a miracle cure, none of the vaccines being deployed in the U.S. or Europe are 100% effective at preventing infection. Additionally varied and evolving strains of COVID complicate the efficacy of the vaccine. There is also the lack of data supporting how long the immunity from the injection lasts.
The U.S. Centers for Disease Control and Prevention says that breakthrough cases are “expected” and that “there will be a small percentage of fully vaccinated people who still get sick, are hospitalized, or die from Covid-19.”
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As of May 1, the CDC said it had “transitioned from monitoring all reported vaccine breakthrough cases to focus on identifying and investigating only hospitalized or fatal cases.” It said the shift would help maximize the quality of the data collected on cases of greatest clinical and public health importance.
Concern was raised over breakthrough Covid cases when preliminary data in Israel, which had one of the fastest vaccination programs in the world, published in July found that the Pfizer–BioNTech Covid-19 vaccine was just 40.5% effective, on average, 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, however, with the vaccine only 16% effective against symptomatic infection for those individuals who had two doses of the shot back in January. But for people that had received two doses by April, the efficacy rate (against symptomatic infection) stood at 79%.
