US CDC: "Vaccine Effectiveness Dropped from 91% to 66% After Delta Became Dominant"

35-Week Longitudinal Study on Medical and Essential Workers

[Image source=Reuters Yonhap News]

[Asia Economy Reporter Cho Hyun-ui] In the United States, the effectiveness of vaccines has sharply declined from the 90% range to the 60% range after the Delta variant became the dominant strain.

According to Bloomberg News on the 24th (local time), the U.S. Centers for Disease Control and Prevention (CDC) stated in a report that "analyzing the genetic sequences of infections among healthcare and essential workers showed a vaccine effectiveness of 66%." This is a 25 percentage point drop from the 91% effectiveness before the Delta variant accounted for more than half of all infections in the U.S.

The researchers tracked and investigated over 4,000 essential workers, including healthcare personnel and rescue workers, from six states over 35 weeks, from December 14 last year to the 14th of this month. Among the total participants, 83% were vaccinated. Of these, two out of three received the Pfizer vaccine, 2% received the Janssen vaccine, and the rest were vaccinated with the Moderna vaccine.

Before the Delta variant became dominant, 10 infections occurred among 2,875 fully vaccinated individuals. However, after the Delta variant became dominant, 24 infections occurred among 2,352 fully vaccinated individuals, reducing the effectiveness to 66%.

Bloomberg News noted that "studies in Israel and the United Kingdom have also shown that the Delta variant reduces vaccine effectiveness."

However, this study has significant statistical uncertainty. After the Delta variant became dominant, vaccine effectiveness was presented with a 95% confidence interval ranging from 26% to 84%. This means that the true vaccine effectiveness value is 95% likely to be between 26% and 84%. Before the Delta variant became dominant, vaccine effectiveness had a 95% confidence interval of 81% to 96%.

Considering the entire study period, among 2,976 fully vaccinated individuals, 34 infections occurred, resulting in an overall vaccine effectiveness of 80%. When calculating vaccine effectiveness only among participants vaccinated within the last 120 days, it was 85%, higher than the 73% effectiveness calculated for those vaccinated more than 150 days ago. However, the researchers explained that this difference was not statistically significant.

This content was produced with the assistance of AI translation services.