COVID-19 mRNA vaccine effectiveness lower to 53% during Delta strain era

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mRNA COVID-19 vaccines provide protection against infection among nursing home residents, but the vaccine effectiveness was lower after the delta variant became the predominant strain, according to research published in the Aug. 18 early-release issue of the U.S. Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report.

Srinivas Nanduri, M.D., from the CDC COVID-19 Response Team, and colleagues analyzed weekly data reported by Centers for Medicare & Medicaid-certified skilled nursing facilities or nursing homes to assess effectiveness of full vaccination with the two currently authorized mRNA COVID-19 vaccines soon after vaccine introduction (March 1 to May 9, 2021; pre-delta period) and when the delta variant predominated (June 21 to Aug. 1, 2021).

The researchers found that the adjusted effectiveness against infection for any mRNA vaccine was 74.7 percent in the pre-delta period using 17,407 weekly reports from 3,862 facilities. During an intermediate period (May 10 to June 20), the adjusted effectiveness was 67.5 percent using 33,160 weekly reports from 11,581 facilities.

During the delta period, the adjusted effectiveness was 53.1 percent using 85,593 weekly reports from 14,917 facilities. Similar effectiveness estimates were reported for the Pfizer-BioNTech and Moderna vaccines.

“To prevent transmission of SARS-CoV-2 in nursing homes, these findings highlight the critical importance of COVID-19 vaccination of staff members, residents, and visitors and adherence to rigorous COVID-19 prevention strategies,” the authors write. “An additional dose of COVID-19 vaccine might be considered for nursing home and long-term care facility residents to optimize a protective immune response.”


Discussion

Analysis of nursing home COVID-19 data from NHSN indicated a significant decline in effectiveness of full mRNA COVID-19 vaccination against laboratory-confirmed SARS-CoV-2 infection, from 74.7% during the pre-Delta period (March 1–May 9, 2021) to 53.1% during the period when the Delta variant predominated in the United States.

This study could not differentiate the independent impact of the Delta variant from other factors, such as potential waning of vaccine-induced immunity. Further research on the possible impact of both factors on VE among nursing home residents is warranted.

Because nursing home residents might remain at some risk for SARS-CoV-2 infection despite vaccination, multipronged COVID-19 prevention strategies, including infection control, testing, and vaccination of nursing home staff members, residents, and visitors are critical.

These results (pre-Delta 74.7%; Delta 53.1%) fall within the range of findings from other studies of COVID-19 mRNA VE in nursing home residents conducted before the Delta variant was prevalent, with estimates against infection ranging from 53% to 92% (3–6). Variability in VE estimates across studies can result from differences in underlying populations, SARS-CoV-2 testing practices and diagnostics, prevalence of previous infections, analytic methods, and virus variant strains in circulation.

Nursing home residents, who are often elderly and frail, might have a less robust response to vaccines, and are at higher risk for infection with SARS-CoV-2 and for severe COVID-19 (8). In addition, nursing home residents were among the earliest groups vaccinated in the United States; thus, if vaccine-induced immunity does wane over time, this decrease in VE might first be observed among nursing home residents.

Because increased U.S. circulation of the Delta variant coincided with a period ≥6 months after vaccine introduction, the extent to which reduced vaccine protection against Delta and potential waning immunity contributed to the lower VE in the Delta period could not be determined by this study.

Nursing homes were aggressive in case ascertainment because of guidelines recommending weekly point prevalence surveys if a single SARS-CoV-2 infection in a staff member or resident was identified.

This analysis assessed VE against any infection, without being able to distinguish between asymptomatic and symptomatic infections. Additional evaluations are needed to understand protection against severe disease in nursing home residents over time.

The findings in this report are subject to at least five limitations. First, resident-level demographic or clinical data were not reported to NHSN. Therefore, the analysis could not control for potential confounders, such as age, presence of underlying health conditions, or the influence of previous SARS-CoV-2 infections on VE.

Second, vaccination dates were not available and time since vaccination could not be measured to evaluate potential waning of protection. Third, staff member vaccination data were not sufficiently complete to assess as a potential confounder.

Fourth, before June 7, 2021, weekly reporting of resident vaccination status was voluntary, and missing data limited inclusion of facility records during this period. Although the magnitude of potential bias introduced by missing data could not be assessed, a bias indicator analysis was conducted, which indicated that VE was likely underestimated during the pre-Delta period (COVID-19 Vaccine Effectiveness Team, CDC, unpublished data, 2021). Finally, the study assessed only nursing home residents and is not generalizable to the broader population.

Both Pfizer-BioNTech and Moderna mRNA vaccines were highly effective in preventing SARS-CoV-2 infection in nursing home residents early after vaccine introduction. However, the effectiveness among this population in recent months has been significantly lower.

To prevent transmission of SARS-CoV-2 in nursing homes, these findings highlight the critical importance of COVID-19 vaccination of staff members, residents, and visitors and adherence to rigorous COVID-19 prevention strategies. An additional dose of COVID-19 vaccine might be considered for nursing home and long-term care facility residents to optimize a protective immune response.***


reference link : https://www.cdc.gov/mmwr/volumes/70/wr/mm7034e3.htm?s_cid=mm7034e3_w

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