New insight into vaccines’ protection against Delta deaths

Jolyon Attwooll

22/10/2021 3:29:11 PM

AstraZeneca and Pfizer offer at least 90% protection from death caused by the Delta variant of the SARS-CoV-2 virus, new international analysis suggests.

AstraZeneca and Pfizer vaccines
More data is emerging on how far vaccines shield recipients from death caused by the Delta variant. (Images: AAP)

An analysis of more than 100,000 COVID-19 cases in Scotland in the New England Journal of Medicine (NEJM) suggests the AstraZeneca vaccine may offer marginally more protection than Pfizer against dying.
The peer-reviewed correspondence from researchers at the University of Edinburgh and the University of Strathclyde was published this week.
The authors looked at positive cases in Scotland from 1 April to 16 August this year recorded in population data covering 5.4 million people, stated in the study as representing around 99% of the entire population of the country.
The cases were tracked up to 27 September. For those who had been vaccinated, the researchers calculated that Pfizer is 90% effective at preventing death among those aged 16 and above, while AstraZeneca is reported as 91% effective.
There were 1,563,818 adults tested in the community during that time, with the mortality analysis based on the 114,706 people among those who tested positive.
All cases were confirmed with a PCR test, with sequencing data showing the Delta variant accounted for the vast majority of cases.
Of the positive cases followed up, 201 people died within the timeframe of the study.
‘In summary, we found that the [Pfizer] and [AstraZeneca] vaccines offered substantial protection against death from COVID-19 caused by the Delta variant,’ the authors wrote.
The authors previously had a study published in The Lancet delving into the efficacy of the vaccines in preventing infection and hospitalisation – but not mortality rates.
‘Both the Oxford–AstraZeneca and Pfizer–BioNTech COVID-19 vaccines were effective in reducing the risk of SARS-CoV-2 infection and COVID-19 hospitalisation in people with the Delta [variant of concern], but these effects on infection appeared to be diminished when compared to those with the Alpha [variant of concern],’ the authors wrote at the time.
The authors say they undertook subsequent research as the data in their original study did not allow them to estimate the vaccines’ effectiveness at preventing death from infection with the Delta variant.
In that first study, they also advised caution in drawing firm conclusions about the relative efficacy of the vaccines due to the research’s observational nature.
For their latest research, COVID-19-related mortalities were recorded among people who had COVID-19 included on their death certificate or those who died within 28 days of a positive test.
People were defined as fully vaccinated if at least 14 days had passed following their second vaccine dose when they had a positive COVID-19 test.
The results were stratified according to age, vaccination status and the vaccine received.
There were no deaths caused by COVID-19 infections among those aged 16–39 who were fully vaccinated. By comparison, there were 17 deaths among unvaccinated people in the same age group.
Meanwhile, Pfizer was found to be more effective at preventing death among 40–59-year-olds. Vaccine efficacy was recorded at 95% for this cohort for Pfizer, compared to 88% for AstraZeneca.
However, for those aged 60 and above, AstraZeneca registered the higher level of protection with 90% vaccine efficacy, against 87% for Pfizer.
The authors note the mortality rate is by far the highest among unvaccinated people aged over 60, with 29.49 deaths per 100,000 person years in this group.
According to tables published with the correspondence, there were 73 deaths among the 5262 cases recorded among those who were fully vaccinated with AstraZeneca, compared to 24 of the 1952 positive cases for those who had two doses of Pfizer.
It meant there were marginally more people dying following both doses of the AstraZeneca compared to those receiving Pfizer (7.5 and 6.84 per 100,000 people years, respectively).
However, once the authors adjusted results to account for age, sex, socioeconomic status and number of relevant coexisting conditions, they found the AstraZeneca vaccine was marginally more effective at stopping deaths.
Another distorting factor included the fact that more than three-quarters of those who were unvaccinated (76.8%) did not have any co-existing clinical conditions when they tested positive for COVID-19, a trend partly reflecting their younger median age.
The study is based on population data and does not consider any variables looking at the potential waning efficacy of either vaccine.
Delta and adolescents
Further peer-reviewed correspondence in the NEJM also reported encouraging results for the Delta variant from a study looking at Pfizer vaccination among those aged 12–18 in Israel.
The authors considered 94,354 adolescents in a vaccinated group, comparing them to an unvaccinated control group of the same size. As with previous studies, it showed the vaccines’ efficacy at preventing symptomatic COVID-19 rising between doses. Researchers found it went up from around 57% after the first dose up to 93% 7–21 days following a second dose.
‘The present observational study provides substantially more precise estimates of vaccine effectiveness among adolescents between the ages of 12 and 18 years for both documented infection and symptomatic disease in a setting in which the Delta variant was predominant,’ the authors write.
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