COVID-19 vaccine effectiveness dropped to 66% against delta, CDC finds

At a community outreach event held in Los Angeles, on Aug. 22, 2021, a nurse gives the Johnson & Johnson COVID-19 vaccination to a man. (Image credit to REDERIC J. BROWN/AFP via Getty Images
A new study by healthcare workers has found that the delta variant has had a devastating effect on COVID-19 vaccine efficacy. It has fallen by approximately 25 percent since it became the dominant coronavirus strain in the U.S.

According to the Centers for Disease Control and Prevention, the study found that vaccine effectiveness against COVID-19 infection declined from 91% before the emergence of the delta variant to 66% after its emergence in the summer.

Health officials noted that despite this "moderate decrease", the "sustained two-thirds reduction" in infection risk "underlines the continued importance of COVID-19 vaccine," they wrote in the study published in the CDC journal Morbidity & Mortality Weekly Report (MMWR).

Related: What is the most deadly coronavirus delta variant of coronavirus?

This study was based on data from over 4,000 health workers from six states in the United States (Arizona Florida, Minnesota Oregon Texas, Texas, Utah, and Oregon). The study spans mid-December 2020 to mid-August 2021. The study showed that the effectiveness of COVID-19 vaccinations against infection was 80% during the period studied (both before the rise of the Delta variant).

Eight-three per cent of the healthcare workers included in the study had been vaccinated. 65% had received Pfizer BioNTech vaccines, 33% received Moderna vaccines and 2% received the Johnson & Johnson vaccine.

Researchers caution that their findings showing a decrease in vaccine effectiveness following the rise of delta should not be taken as gospel. This is because vaccine effectiveness could also be decreasing due to the longer time between vaccinations. This means that people's immunity could have decreased slightly over time rather than being the result of a new vaccine.

Also, the study did not assess the severity of the infections. A second CDC study was published in MMWR on Tuesday. It examined the severity of COVID-19 in Los Angeles by comparing hospitalizations between unvaccinated and vaccinated patients.

The second study examined over 43,000 COVID-19 infection among L.A. residents between May 1 and July 25, 2021. It found that 71.4% were unvaccinated, 25.3% were fully vaccinated, and 3.3% were partially vaccinated. The COVID-19 infection rate in unvaccinated individuals was almost 5 times higher than that of fully vaccinated persons, with a hospitalization rate nearly 30 percent higher.

The authors stated that these hospitalization and infection rate data show that vaccines approved were effective in protecting against SARS-CoV-2 and severe COVID-19 infections during an increase in transmission of the Delta variant. To prevent COVID-19-related deaths and hospitalizations, it is important to coordinate efforts to increase COVID-19 vaccine.

Original publication on Live Science