Public Health

Transmission of Covid-19 post vaccination


In December I reviewed two papers published that month (Ref 1). One was in the New England Journal of Medicine by Polack et al. It was called "Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine" (Ref 2). The other was the Voysey et al paper in The Lancet called "Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK" (Ref 3). The first presented the initial safety and efficacy findings for the Pfizer Covid-19 vaccine; the second presented the same for the Oxford AstraZeneca vaccine.

An important BMJ article was referenced in that post (Ref 4). It was written by Dr Peter Doshi and it summarised some aspects of the vaccine trials of which I had not been aware, and I suspect many others had not been aware either:

None of the trials currently under way are designed to detect a reduction in any serious outcome such as hospital admissions, use of intensive care, or deaths. Nor are the vaccines being studied to determine whether they can interrupt transmission of the virus.”

Table 1 in Doshi’s paper summarised characteristics of ongoing Phase 3 Covid-19 vaccine trials for Moderna, Pfizer, AstraZeneca (US), AstraZeneca (UK), Janssen, Sinopharm, and Sinovac. All the trials excluded children, adolescents, immunocompromised patients, pregnant or breastfeeding women. None were designed to evaluate reduction in severe cases (hospitalisation, intensive care (ICU), or death) and none were designed to evaluate interruption of transmission (person to person spread.) This does not seem to be widely known.

The trials were set up to measure cases, defined as a positive PCR test with at least one symptom of Covid-19. Cases were the primary outcome reported in the results, which led to the medications being approved for emergency use, while trials are ongoing. (The Pfizer trial is not due for completion until April 6th, 2023 (Ref 5) and the Oxford/AstraZeneca trial is not due for completion until February 14th, 2023 (Ref 6).)

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