Hopes for COVID-19 vaccines to beat back the pandemic assumed mass-vaccination would slow viral transmission, and not just prevent moderate-to-severe disease.
Now, early data suggest that AstraZeneca’s (LON:AZN) COVID-19 vaccine, which it jointly developed with Oxford University, can curb viral transmission by up to 67%. The researchers didn’t study transmissibility per se; the inferred it by taking weekly PCR swab tests from volunteers. The first dose led to the 67% reduction in PCR positivity while the second dose led to a 50% reduction.
There was more good news from the data. The ChAdOx1 vaccine offered 100% protection against severe disease in a Phase 3 trial. A single vaccine dose offered 76% efficacy between 22 and 90 days after vaccination. Delaying the second dose of vaccine by 12 weeks appeared to boost efficacy to 82%. Efficacy fell to 55% when the gap between doses dropped to less than six weeks.
The study concluded that the U.K.’s strategy of aiming to vaccinate a large proportion of the population with the first dose before administering the second vaccine dose up to 12 weeks later is “an effective strategy.”
Also heartening is the fact that a portion of the data came from regions with highly transmissible variants, including Brazil (Phase 3) and South Africa (Phase 2).
While a single vaccine dose was effective at engendering short-term immunity, vaccine efficacy fell to 32% between 90 and 120 days after administration of a single-dose shot. “A second dose of ChAdOx1 nCoV-19 induces increased neutralising antibody levels and is likely necessary for long lasting protection,” the researchers concluded in the preprint article in The Lancet.
The U.S. has not yet authorized the AstraZeneca vaccine as the FDA is awaiting more results from a Phase 3 clinical trial, which are due later this month.