Findings from a large UK study suggest that while Pfizer and BioNTech's COVID-19 vaccine Comirnaty is initially more effective against the highly contagious Delta variant than AstraZeneca's Vaxzevria, this protection then declines at a quicker rate. Nevertheless, researchers at the University of Oxford who ran the study stressed that obtaining two vaccine doses remains the best way to ensure protection against Delta infections. The results were released as a pre-print on Thursday.
The work looks at data between December 2020 and August 2021 from the UK's ongoing COVID-19 Infection Survey, which is comparing real-world effectiveness of coronavirus vaccines against SARS-CoV-2 infections, including asymptomatic cases. Over 3 million swab tests from more than 740,000 participants were analysed before and after May 17, when Delta became the main variant in the UK.
Comirnaty had 93% effectiveness against symptomatic infection two weeks after the second dose, compared with 71% for Vaxzevria, although Comirnaty's effectiveness dropped over time, whereas Vaxzevria's remained largely the same. For infections with a high viral load, the analysis showed that protection a month after the second Comirnaty dose was 90% greater than an unvaccinated individual, slipping to 85% after two months and 78% after three. Protection conferred by Vaxzevria was 67%, 65% and 61%, respectively, researchers said. Results suggest that after four to five months, effectiveness of these two vaccines would be similar, but long-term outcomes need to be studied, they added.
Investigators also found that Delta infections after two vaccine doses had similar peak levels of virus to those in unvaccinated people. By contrast, viral loads in those infected post-vaccination were much lower with the Alpha variant.
Oxford researcher Sarah Walker, academic lead for the COVID-19 Infection Survey, said "we don't yet know how much transmission can happen from people who get COVID-19 after being vaccinated – for example, they may have high levels of virus for shorter periods of time – but the fact that they can have high levels of virus suggests that people who aren't yet vaccinated may not be as protected from the Delta variant as we hoped."
However, Koen Pouwels, a senior researcher at the university, noted that "even with these slight declines in protection against all infections and infections with high viral burden, it's important to note that overall effectiveness is still very high because we were starting at such a high level of protection."
Meanwhile, Oxford said the findings suggest Moderna's vaccine Spikevax may offer better protection when it comes to preventing infections caused by the Delta variant. The researchers noted that a single dose of Spikevax has "similar or greater effectiveness" against Delta as single doses of the other vaccines. They pointed out that there is insufficient data on second doses of Moderna's jab – which like Comirnaty is based on mRNA, while Vaxzevria is a viral vector vaccine – but believe it is "almost certainly at least as good as the others."
Recent non-peer-reviewed results from a Mayo Clinic study appear to give Moderna's vaccine an edge as well. It found that while Spikevax and Comirnaty, also known as mRNA-1273 and BNT162b2, respectively, both offered good protection against infections and hospitalisations, both saw their efficacy rates decline against Delta, although the drop with Moderna's vaccine was notably less pronounced.
Pfizer and BioNTech believe a third dose of their vaccine will prolong immunity against COVID-19, and also boost protection against variants of concern such as Delta. They submitted early data to the FDA this week to back up their claim. Meanwhile, the US is preparing for a broad rollout of third doses of mRNA COVID-19 vaccines beginning the week of September 20. Under the expanded plan, individuals would be offered a booster eight months after their second dose.
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