Protection significantly lower after one dose, study suggests

Covid-19 vaccines were estimated to be highly effective against the delta variant of the SARS-CoV-2 virus following full vaccination in a test-negative case-control study from the United Kingdom, with partial vaccination conveying less protection against the variant.

In the study of two vaccines widely used in the U.K.—the BNT162b2 messenger RNA vaccine and ChAdOx1 nCoV-19 adenoviral vector vaccine—efficacy for both vaccines after a single dose was found to be significantly lower against the delta variant than the alpha variant (30.7% versus 48.7%).

Among people who were fully vaccinated (two doses), the efficacy of the mRNA vaccine and adenoviral vector vaccine, respectively, was found to be 88% and 67% against the delta variant.

The delta variant is now the dominant strain of SARS-CoV-2 worldwide, and it is being blamed for a 70% rise in Covid-19 cases and a more than 25% increase in deaths in the United States in recent weeks.

The study findings, published online July 21 in The New England Journal of Medicine, show high levels of vaccine efficacy against symptomatic disease involving the delta variant after full vaccination, with only “modestly lower” effectiveness than that estimated against the alpha variant, wrote researcher Jamie Lopez Bernal, PhD, of Public Health England, and colleagues.

“Our finding of a reduced effectiveness after the first dose would support efforts to maximize vaccine uptake with two doses among vulnerable groups in the context of circulation of the delta variant,” they added.

The researchers used two approaches to estimate the efficacy of Covid-19 vaccination against the delta variant. Data on all symptomatic sequenced cases of Covid-19 in England were used to estimate the proportion of cases with either variant according to patient vaccination status.

A test-negative case-control design was employed to estimate vaccine efficacy against symptomatic disease caused by the delta variant, as compared with the alpha variant, over the period that the delta variant has been circulating in the U.K.

In a secondary analysis, the researchers estimated the proportion of people with Covid-19 caused by the delta variant relative to the alpha variant—which was the dominant variant at the time of the study—according to vaccination status.

“The underlying assertion was that if the vaccine had some efficacy and was equally effective against each variant, a similar proportion of cases with either variant would be expected in unvaccinated persons and in vaccinated persons,” the researchers wrote.

Conversely, they noted that if the vaccine was less effective against the delta variant than the alpha variant, the delta variant would make up a higher proportion of cases occurring more than 3 weeks after vaccination than among unvaccinated people.

The researchers included data on people in England vaccinated up until May 16 of this year.

Among their main findings:

  • Efficacy following one dose of either the mRNA or viral vector vaccine was notably lower among people with the delta variant (30.7%; 95% CI< 25.2-35.7) compared to those with the alpha variant (48.7%; 95% CI, 45.5-51.7), with similar efficacy seen for both vaccines.
  • Full vaccination effectiveness with the mRNA vaccine was 93.7% (95% CI, 91.6-95.3) among people with the alpha variant and 88.0% (95% CI, 85.3-90.1) among those with the delta variant.
  • With the adenoviral vector vaccine, the effectiveness of two vaccine doses was 74.5% (95% CI, 68.4-79.4) among people with the alpha variant and 67.0% (95% CI, 61.3-71.8) among those with the delta variant.

In commentary published with the study, Stephen Evans, MSc, and Nicholas Jewell, PhD, of the London School of Hygiene and Tropical Medicine, noted that several Covid-19 vaccine manufacturers have recently released data showing their vaccine’s efficacy against the delta variant, but Evans and Jewell wrote that “these results are based on small samples and need confirmation by valid observational data from large samples of vaccinated persons.”

They added that a test-negative design study from Canada, recently published prior to peer review, showed efficacy against symptomatic disease caused by the alpha, beta, gamma and delta variants for the BNT162b2 and mRNA-1273 vaccines as well as the ChAdOx1 nCoV-19 vaccine.

“As of June 19, 2021, Covid-19 vaccines are estimated to have prevented 7.2 million infections and 27,000 deaths in England alone. Similarly, in the United States, an estimated 279,000 deaths and up to 1.25 million hospitalizations have been averted as of the end of June 2021,” they wrote. “As the delta variant affects various countries, including the United States, the current imperative is to vaccinate as many people as possible, as quickly as possible. Under served and at-risk communities must not be neglected when implementing this strategy.”

  1. Among people who were fully vaccinated, mRNA vaccine and adenoviral vector vaccine efficacy, respectively, was found to be 88% and 67% against the delta variant.

  2. In the study of two vaccines widely used in the U.K.—the BNT162b2 messenger RNA vaccine and ChAdOx1 nCoV-19 adenoviral vector vaccine—efficacy for both vaccines after a single dose was found to be significantly lower against the delta variant than the alpha variant (30.7% versus 48.7%).

Salynn Boyles, Contributing Writer, BreakingMED™

This research was funded by Public Health England. Corresponding researcher Jamie Lopez Bernal reported no relevant disclosures. Commentary writer Nicholas Jewell reported receiving grants from the University of California, Berkeley.

Cat ID: 190

Topic ID: 79,190,933,190,31,926,192,561,927,151,928,925,934