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COVID-19 Resource Center

Disclosures: Adalja, Fowlkes and Griffin report no relevant financial disclosures. Please see the studies for all other authors’ relevant financial disclosures.
August 26, 2021
2 min read

Vaccines less effective in face of delta, remain protective against serious illness

Disclosures: Adalja, Fowlkes and Griffin report no relevant financial disclosures. Please see the studies for all other authors’ relevant financial disclosures.
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COVID-19 vaccine effectiveness declined to 66% among a cohort of health care workers after delta became the predominant SARS-CoV-2 variant, according to a report in MMWR.

However, a second report in the journal showed that unvaccinated people in Los Angeles County were around five times more likely to be infected and approximately 29 times more likely to be hospitalized with COVID-19 than vaccinated people.

Amesh A. Adalja

“Vaccines are not bug zappers, and the efficacy that really matters is not against any infection but against severe infection,” Amesh A. Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security, told Healio.

“By that standard,” Adalja said, “the vaccines are performing extremely well, even in the face of the delta variant. Chasing mild and/or clinically insignificant cases that occur after vaccination is of marginal value.”

In the first study, Ashley Fowlkes, ScD, an epidemiologist on the CDC COVID-19 Response Team, and colleagues tested health care personnel, first responders and other essential workers from eight locations in six U.S. states weekly for COVID-19 from Dec. 14, through April 21.

Of a total of 4,217 participants, 3,483 (83%) were fully vaccinated. Of those who were vaccinated, 2,278 (65%) received the Pfizer-BioNTech vaccine, 1,138 (33%) received the Moderna vaccine and 67 (2%) received the Johnson & Johnson shot.

According to Fowlkes and colleagues, the adjusted vaccine effectiveness against COVID-19 dropped to from 91% (95% CI, 81%-96%) during the months before the delta variant predominated to 66% (95% CI, 26%-84%) during the period of delta predominance.

“This trend should be interpreted with caution because [vaccine effectiveness] might also be declining as time since vaccination increases and because of poor precision in estimates due to limited number of weeks of observation and few infections among participants,” the authors wrote.

In the second study, Jennifer B. Griffin, PhD, supervising epidemiologist at the Los Angeles County Department of Public Health, and colleagues assessed rates of infection and hospitalization among people aged 16 years or older from May 1 through July 25 and found that, during the study period, COVID-19 cases caused by the delta variant increased from 8.6% to 91.2% in fully vaccinated individuals, from 0% to 88.1% in partially vaccinated people, and from 8.2% to 87.1% in those who were not vaccinated.

Among 43,127 cases of SARS-CoV-2 infection, 10,895 (25.3%) occurred in fully vaccinated people, 1,431 (3.3%) in partially vaccinated people and 30,801 (71.4%) in unvaccinated participants.

“On July 25, the SARS-CoV-2 infection rate among unvaccinated persons was 4.9 times and the hospitalization rate was 29.2 times the rates among fully vaccinated persons,” they reported.

Among those who were fully vaccinated, 3.2% were hospitalized, 0.5% were admitted to the ICU and 0.2% required mechanical ventilation, according to the researchers. Among those who were partially vaccinated, 6.2% were hospitalized, 1% were admitted to the ICU, and 0.3% needed ventilation. Among unvaccinated individuals, 7.6% required hospitalization, 1.5% were admitted to the ICU, and 0.5% required ventilation.

“The findings in this report are similar to those from recent studies indicating that COVID-19 vaccination protects against severe COVID-19 in areas with increasing prevalence of the SARS-CoV-2 delta variant,” Griffin and colleagues wrote. “Efforts to increase COVID-19 vaccination coverage, in coordination with other prevention strategies, are critical to preventing COVID-19-related hospitalizations and deaths.”


Fowlkes A, et al. Morb Mortal Wkly Rep MMWR. 2021;doi:10.15585/mmwr.mm7034e4.

Griffin JB, et al. Morb Mortal Wkly Rep MMWR. 2021;doi:10.15585/mmwr.mm7034e5.