A US study calculated COVID-19 vaccine effectiveness to be 80% in a large group of fully vaccinated frontline workers between December 2020 and August 2021, compared with 91% in earlier surveys. The estimates were based on COVID-19 RT-PCR testing and did not measure whether there were changes in efficacy in protecting against severe disease, including hospitalisation and death.
The authors say one reason for the change could be waning immunity, a decrease in the strength of the body's vaccine-activated defences against the virus. The difference may also reflect the fact that the vaccines are not as effective against the highly contagious Delta variant of the SARS- CoV-2 virus, which since June 2021, has become the most common cause of COVID-19 in the US.
“The vaccines are still helping save lives and keep people from getting sick, despite a slight diminishing return over many months,” explains Matthew Thiese, associate professor at the University of Utah Rocky Mountain Center for Occupational and Environmental Health (RMCOEH). “These data, combined with other data, demonstrate that vaccinated people are much less likely to get COVID-19 and are much less likely to be hospitalised.”
Thiese is co-investigator and RMCOEH assistant professor Sarang Yoon, D.O., is primary investigator of the HEROES- RECOVER (Research on the Epidemiology of SARS-CoV-2 in Essential Response Personnel) study site in Utah.
The study was published on 24 August in the Morbidity Mortality Weekly Report (MMWR) from the Centers for Disease Control and Prevention (CDC). Additional study sites in the HEROES-RECOVER network are Phoenix, Tucson, and other areas in Arizona; Miami, Florida; Portland, Oregon; Duluth, Minnesota; and Temple, Texas.
The network followed 4,136 health care personnel, first responders, and essential workers who had not previously had COVID-19. Study participants submitted samples for RT-PCR testing on a weekly basis and 2,976 participants were fully vaccinated within the study period, receiving either the Pfizer-BioNtech (65%), Moderna (33%), or Johnson & Johnson (2%) vaccines. Test results from these groups between 14 December 2020 to 14 August 2021 show that:
* Among unvaccinated study participants, 194 infections occurred in 181,357 person-days (combined total of number of days of testing for this group). Among fully vaccinated participants, 34 infections occurred in 454,832 person-days.
* During that time period, the vaccines were 80% effective for all fully vaccinated study participants, but preliminary data indicate that vaccines may wane in intensity over time, with lower effectiveness after five or more months following full vaccination. In addition, the vaccines appeared to be less effective during the last 43 days of the study period when Delta became the predominant virus variant. However, because sample sizes were small, these results were not statistically significant. These trends will be investigated further in future studies.
Study results
Effectiveness of COVID-19 Vaccines in Preventing SARS-CoV-2 Infection Among Frontline Workers Before and During B.1.617.2 (Delta) Variant Predominance — Eight U.S. Locations, December 2020–August 2021
Ashley Fowlkes, Manjusha Gaglani, Kimberly Groover, Matthew S. Thiese, Harmony Tyner, Katherine Ellingson, HEROES-RECOVER Cohorts
Published in CDC Morbidity and Mortality Weekly Report on 27 August 2021
During December 14, 2020–April 10, 2021, data from the HEROES-RECOVER Cohorts,* a network of prospective cohorts among frontline workers, showed that the Pfizer-BioNTech and Moderna mRNA COVID-19 vaccines were approximately 90% effective in preventing symptomatic and asymptomatic infection with SARS-CoV-2, the virus that causes COVID-19, in real-world conditions. This report updates vaccine effectiveness (VE) estimates including all COVID-19 vaccines available through August 14, 2021, and examines whether VE differs for adults with increasing time since completion of all recommended vaccine doses. VE before and during SARS-CoV-2 B.1.617.2 (Delta) variant predominance, which coincided with an increase in reported COVID-19 vaccine breakthrough infections, were compared.
Methods for the HEROES-RECOVER Cohorts have been published previously. Health care personnel, first responders, and other essential and frontline workers in eight locations across six states were tested weekly for SARS-CoV-2 infection by reverse transcription–polymerase chain reaction (RT-PCR)† and upon the onset of any COVID-19–like illness. Weeks when the Delta variant accounted for ≥50% of viruses sequenced, based on data from each respective location, were defined as weeks of Delta variant predominance.
Vaccination was documented by self-report and verified by provision of vaccine cards or extraction from electronic medical records or state immunization registries. Among 4,217 participants, 3,483 (83%) were vaccinated; 2,278 (65%) received Pfizer-BioNTech, 1,138 (33%) Moderna, and 67 (2%) Janssen (Johnson & Johnson) COVID-19 vaccines. Cox proportional hazards models were used to calculate ratios of unvaccinated to fully vaccinated (≥14 days after receipt of all recommended COVID-19 vaccine doses) infection rates, adjusted for occupation, site, and local viral circulation, and weighted for inverse probability of vaccination using socio-demographic characteristics, health information, frequency of close social contact, and mask use. This activity was reviewed by CDC and was conducted consistent with applicable federal law and CDC policy.
During the 35-week study period, 4,136 participants with no previous laboratory-documented SARS-CoV-2 infection contributed a median of 20 unvaccinated days per participant (interquartile range [IQR] = 8–45 days; total = 181,357 days), during which 194 SARS-CoV-2 infections were identified; 89.7% of these infections were symptomatic. A total of 2,976 participants contributed a median of 177 fully vaccinated days (IQR = 115–195 days; total = 455,175 days) with 34 infections, 80.6% of which were symptomatic. Adjusted VE against SARS-CoV-2 infection was 80% (95% confidence interval [CI] = 69%–88%). The VE point estimate was 85% among participants for whom <120 days had elapsed since completion of full vaccination compared with 73% among those for whom ≥150 days had elapsed; however the VE 95% CI were overlapping, indicating the difference was not statistically significant.
During Delta variant–predominant weeks at study sites, 488 unvaccinated participants contributed a median of 43 days (IQR = 37–69 days; total = 24,871 days) with 19 SARS-CoV-2 infections (94.7% symptomatic); 2,352 fully vaccinated participants contributed a median of 49 days (IQR = 35–56 days; total = 119,218 days) with 24 SARS-CoV-2 infections (75.0% symptomatic). Adjusted VE during this Delta predominant period was 66% (95% CI = 26%–84%) compared with 91% (95% CI = 81%–96%) during the months preceding Delta predominance.
During 14 December 2020–14 August 2021, full vaccination with COVID-19 vaccines was 80% effective in preventing RT-PCR–confirmed SARS-CoV-2 infection among frontline workers, further affirming the highly protective benefit of full vaccination up to and through the most recent summer US COVID-19 pandemic waves. The VE point estimates declined from 91% before predominance of the SARS-CoV-2 Delta variant to 66% since the SARS-CoV-2 Delta variant became predominant at the HEROES-RECOVER cohort study sites; however, this trend should be interpreted with caution because VE 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.
As with all observational VE studies, unmeasured and residual confounding might be present. Active surveillance through the cohort is ongoing and VE estimates will be monitored continuously. 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.
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