As Delta spread, Covid-19 vaccine effectiveness against infection fell from 90% to 66% in one key study

The effectiveness of Covid-19 vaccines at preventing infection fell in one study of U.S. frontline workers from roughly 90% to 66% as the Delta variant emerged and became dominant in the country, an updated report from the Centers for Disease Control and Prevention released Tuesday indicated.

The study, known as HEROES-RECOVER, includes more than 4,000 health care workers, first responders, and other frontline workers in eight locations across six states, all of whom have been tested weekly for infection with SARS-CoV-2. More than 4 in 5 were vaccinated, and the vast majority of them received the mRNA vaccines from either Pfizer-BioNTech or Moderna.

Researchers had previously reported that from Dec. 14, 2020, when the vaccines started rolling out, to April 10, 2021, the vaccines were approximately 91% effective at preventing both symptomatic and asymptomatic infection from SARS-2. The finding underlined the ability of the vaccines to halt infection entirely and slash transmission; the clinical trials that led to the authorization of the vaccines had generally focused on their effectiveness at preventing symptomatic Covid-19 cases.

That study, however, played out before the Delta variant spread widely in this country. In the more recent time frame, which accounts for data up through Aug. 14, vaccine effectiveness fell to 66%, researchers reported in the study, which was published in the CDC’s Morbidity and Mortality Weekly Report.

Experts caution against relying solely on one real-world study in determining effectiveness against infection. But other studies trying to establish the effectiveness of the vaccines at preventing infection have also generally shown declines against Delta, with some estimates in the same ballpark as the new CDC study.

The study’s findings fit with the overall narrative of the Delta era of the pandemic.

In the spring, as it became clear that the vaccines were preventing the vast majority of infections, the CDC advised that vaccinated people no longer needed to wear masks. But as Delta took off, it became clear that Delta was causing breakthrough infections — as well as symptomatic breakthrough infections — at higher rates than other forms of the virus.

Scientists also concluded that vaccinated people who were still infected with Delta could transmit the virus to others (though they are not as infectious as unvaccinated people who contract Delta). The findings led to the CDC’s decision last month to roll back its mask guidance and encourage even vaccinated people to wear them in areas of the country with high rates of transmission.

In the new report, the researchers noted that a 66% effectiveness rate still showed that the vaccines were protecting lots of people from contracting the virus. “Although these interim findings suggest a moderate reduction in the effectiveness of Covid-19 vaccines in preventing infection, the sustained two thirds reduction in infection risk underscores the continued importance and benefits of Covid-19 vaccination,” they wrote.

There are several factors that could be influencing the decline in reported effectiveness, experts say. For one, Delta is more capable of causing breakthrough infections than other forms of the virus. But some waning in the power of immune systems to block infections could be occurring as well, the authors note. Experts have also pointed out that, as mitigation efforts have been eased, social contact among individuals has increased, and transmission has picked up. People are more likely to be exposed to the virus than they were in the early days of the vaccine rollout.

The updated study notably did not look at effectiveness at preventing severe disease or death. Other studies have shown that even in the face of Delta, the vaccines are maintaining their protectiveness against the worst outcomes of the infection. Delta, it appears, can overwhelm the initial immune response and start an infection, but vaccinated individuals can fend it off before it causes serious damage. The maintained strength of the vaccines has led many scientists to push back on the Biden administration’s plans to distribute boosters to the general population starting this fall.

Another study published Tuesday in the same MMWR issue indicated that from May through July in Los Angeles County, unvaccinated people had five times the rate of Covid-19 infections as vaccinated people and were 29 times more likely to be hospitalized.