COVID-19 vaccine effectiveness barely affected by Delta variant, research finds

Did the Delta variant (B.1.617.2) reduce the efficacy of the vaccine? If you ask Ian Foster of the Argonne National Laboratory and his colleagues, the answer is not really. New research published in the prepublication medRxiv* The server suggests that vaccines approved in the United States are stable and effective.

Delta overtook the Alpha variant as the most dominant severe acute respiratory virus 2 (SARS-CoV-2) in the United States. It caused an uproar of concern in late summer 2021 after reports of major infections in vaccinated individuals.

The United States remains the country with the highest number of cases and deaths from coronavirus 19 (COVID-19) infection. In the past 28 days, there have been over 4 million cases of coronavirus and over 48,000 deaths.

The researchers suggest that vaccines are an excellent measure of protection against Delta because the Delta variant is unlikely to significantly evade vaccine-induced immunity. In addition, the differences in the effectiveness of vaccines are likely due to the age of the people vaccinated.

“We conclude from this data that there is no evidence that the Delta variant escapes immunity from COVID-19 vaccines used in the United States, and therefore there is no evidence that the variant causes additional infections, beyond infections already expected due to the known imperfect efficacy and imperfect efficacy of available COVID-19 vaccines, ”the team wrote.

Study details

The researchers used data sources from public health authorities in 7 states, 5 counties and the District of Columbia to collect the COVID-19 test results. They grouped the results according to vaccination status between May 15 and September 15, 2021.

The period was chosen because the Delta variant has become more prevalent in the United States. As a result, its transmission across the country reached almost 100% at the end of August 2021.

All vaccine efficacy data, subjected to a joint adjustment weighted by Bayesian errors, gives an overall efficacy of 84.1%.

Vaccine efficacy did not decline in the presence of the Delta variant

The vaccine’s efficacy remained high in the United States from mid-May to the end of August 2021.

Only Washington DC showed a slight drop in vaccine effectiveness in late June, although that variation may come from weak testing in the region.

From early to mid-July, there was a slight drop in vaccine effectiveness in almost all of the areas studied. However, the decline in vaccine efficacy was recovered in early September.

Among the individual US counties plus Washington DC, there is no downward trend in vaccine efficacy that would be expected if the Delta variant were able to escape the antibodies induced by the vaccine. “Instead, the data shows [vaccine effectiveness] fluctuations, some upward, some downward, some even oscillating, and these are probably due to demographic and environmental factors affecting vaccination statistics… ”, explained the researchers.

State-level data showed a similar aspect: no continued decrease in vaccine effectiveness was observed.

Age-related differences in vaccine cohorts

Researchers categorized data on vaccinated populations by age 12 to 18, 18 to 65, and 65 and over. This allowed them to look at the average age of the person vaccinated.

The average age of the vaccinated person dropped to four years and three months during the summer of 2021. Thus, the effectiveness of the vaccine likely increased as more younger adults were vaccinated during the spread of. Delta.

The researchers suggest that the increased vaccination rate over the summer may have contributed to the “recovery” of the vaccine’s effectiveness later.

While age appears to play a potential role in vaccine effectiveness, other factors may be involved. Further research examining changes in the demographics of vaccinated and unvaccinated cohorts, changes in social behavior, or environmental effects is needed to provide a better explanation for small and temporary fluctuations in vaccine efficacy.

*Important Notice

medRxiv publishes preliminary scientific reports which are not peer reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.