Snow Leopard
Senior Member (Voting Rights)
So are you expecting the vaccine to reduce transmissibility from people who have been vaccinated, even if they get infected? There seems to be such doubt about this in the media.
Every other vaccine has been tested on the basis of reducing symptoms, rather than reducing transmission. It just so happens that they're effective at doing the latter too.
None of the phase 3 vaccine trials are sufficiently powered to get a true idea about transmission rates - they'd need ten times as many participants, or much longer followup periods (the latter of which is probably unethical since the vaccines have demonstrated to be effective).
But we can guess.
The AstraZeneca vaccine (interim) data published in The Lancet showed that 14 days after the second dose, the efficacy was 55.7% for any positive result, including asymptomatic cases, compared to 70.4% against symptomatic cases. It is not unreasonable to expect a similar reduction against asymptomatic infections in the Pfizer/Moderna vaccines.
Note that a true 80% effective vaccine will need 75% of the population to achieve 'herd immunity, and a true 60% vaccine will need 100% to be vaccinated. See: https://www.medrxiv.org/content/10.1101/2020.12.15.20248278v2.full-text
It is debatable how effective the vaccine will be in reducing transmission from symptomatic cases. But no one should argue that someone who has no symptoms and tests negative (on PCR) is going to be infectious.
Which leads to the last topic - very few (natural) COVID cases are asymptomatic and asymptomatic spread is rare.
Household transmission of SARS-CoV-2 and risk factors for susceptibility and infectivity in Wuhan: a retrospective observational study
https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30981-6/fulltext
This study is based on a 'whole' set of contact tracing data between specific dates in Wuhan.
Looking at the raw numbers, 2% of the overall infections were reported as asymptomatic. Of the secondary infections only 0.3% were due to exposure to asymptomatic cases.
Given the difference in rate of infection in the different clinical severity groups, it may be reasonable to suggest that there could be a reduction in transmissibility even in the mild symptomatic (post-vaccination) cases compared to a naturalistic case (which may be more severe). In terms of asymptomatic cases, I'd expect at least a 90% drop in transmissibility, compared to a naturalistic unvaccinated COVID case, given the above data.
If there is a 25% drop in transmissibility in symptomatic cases post-vaccination a 90% drop in transmissibility for asymptomatic cases and an (overall) 15% efficacy gap between asymptomatic and symptomatic cases, then the efficacy against transmission of the Pfizer/Moderna drops to around 83% (from 95%) and the AZ vaccine drops to around 65% (from 70.4%).
These are not concrete numbers, but illustration of what could happen given the hypothetical risk of asymptomatic case transmission.