Pfizer, AstraZeneca vaccines effective against severe COVID-19, UK variant

Getting vaccinated with one dose of either the BNT162b2 (Pfizer-BioNTech) or ChAdOx1-S (Oxford-AstraZeneca) results in a significant decrease in symptomatic COVID-19 in older adults and in further protection against severe disease, according to a recent study in England. In addition, the Pfizer-BioNTech vaccine has shown efficacy against the B.1.1.7 or the UK variant.

“We found a clear effect from the first dose of the BNT162b2 and ChAdOx1-S vaccines, and … a large effect of a single dose against severe outcomes of COVID-19–related hospital admissions and mortality, supporting the decision to maximize the number of individuals vaccinated with a single dose—although we have limited evidence on the duration of this effect,” the researchers said.

A total of 156,930 adults aged ≥70 years who reported symptoms of COVID-19 between 8 December 2020 and 19 February 2021 were included in this test negative case-control study, and were successfully linked to vaccination data in the National Immunisation Management System. Participants were vaccinated with either BNT162b2 or ChAdOx1-S.

Individuals aged ≥80 years who were vaccinated with Pfizer-BioNTech before 4 January 2021 were more likely to test positive for COVID-19 in the first 19 days after vaccination (odds ratio [OR] up to 1.48, 95 percent confidence interval [CI], 1.23–1.77), suggesting that those initially targeted had a higher underlying risk of infection. [BMJ 2021;373:n1088]

Then, vaccine effectiveness was compared with the baseline postvaccination period. Vaccine effects were noted 10 to 13 days after vaccination, achieving an effectiveness of 70 percent (95 percent CI, 59–78) before plateauing. From 14 days after vaccination with the second dose, effectiveness reached 89 percent (95 percent CI, 85–93) relative to the increased baseline risk.

The effectiveness of the BNT162b2 vaccine reached 61 percent (95 percent CI, 51–69) from 28 to 34 days postvaccination, then plateaued. With ChAdOx1-S, effects were noted 14 to 20 days after vaccination, with an effectiveness of 60 percent (95 percent CI, 41–73) from 28 to 34 days, increasing to 73 percent (95 percent CI, 27–90) from day 35 onwards.

In addition to the protective benefit of vaccines against symptomatic COVID-19, participants who had received one dose of BNT162b2 had a further 43-percent (95 percent CI, 33–52) reduced risk of emergency hospital admission and 51-percent (95 percent CI, 37–62) lower risk of death. Those who had received a single dose of ChAdOx1-S had a further 37-percent (95 percent CI, 3–59) reduced risk of emergency hospital admission. Follow-up was insufficient to estimate the effect of ChAdOx1-S on death.

“Combined with the effect against symptomatic disease, a single dose of either vaccine was about 80-percent effective at preventing admission to hospital with COVID-19 and a single dose of BNT162b2 was 85-percent effective at preventing death with COVID-19,” the researchers said.

UK variant

In analysis by variant based on spike gene target failure, there was little difference in effects by variant. Such finding was consistent with recent evidence showing that sera from vaccinated individuals elicited equivalent neutralizing titres to the B.1.1.7 variant and similar variants to that seen with previous strains. [bioRxiv 2021:2021.01.07.425740; bioRxiv 2021:2021.01.18.426984]

Real-world evidence on the early effectiveness of a single-dose of BNT162b2 has also been reported in Israel. Chodick and colleagues estimated a vaccine effectiveness of 52 percent during the first 24 days postvaccination. However, a reanalysis of the same data by Hunter and colleagues reported an effectiveness of 90 percent by day 24. [medRxiv 2021:2021.01.27.21250612; medRxiv 2021:2021.02.01.21250957]

In a linked editorial, Andreas Martin Lisewski from the Department of Life Sciences and Chemistry/Focus Area Health, Jacobs University Bremen, in Germany took notice of the increased odds of a positive test result within 20 days of an initial dose of Pfizer’s vaccine among adults aged ≥80 years, which deserved further explanation. [BMJ 2021;373:n1201]

“The authors suggest that this increased risk was probably due to the selection of high-risk individuals for vaccination during the campaign’s earliest stages,” Lisewski said.

“If so, the effect size would be expected to decrease steadily with time, but instead the OR for a positive test result first increased to a peak of 1.47 (95 percent CI, 1.23–1.76) after 9 days, and then decreased back to baseline (OR, 1.00) before 21 days postvaccination. This pattern remains unexplained,” he added.

“Further evidence is needed on the duration of any effect and the effect against asymptomatic infection and transmission, and the four UK nations will work closely to develop and share evidence on this as it becomes available,” the researchers of the current study said.

“Nevertheless, the fact that the vaccine appears to be preventing symptomatic disease, including with the B.1.1.7 variant, is encouraging, and this is likely to have an important impact on the detection of people with COVID-19 and severe outcomes at a population level,” they added.