It's being widely reported that 1 dose of the Astra/Oxford vaccine reduces transmission by 67%. My understanding is that this is not what the paper shows, or what the authors have claimed. I'm happy to be corrected if I've got this wrong. Short thread. https://www.theguardian.com/society/2021/feb/02/single-dose-of-astrazeneca-vaccine-could-cut-transmission-by-67

The Astra/Oxford trial asked people enrolled to self-swab every week and send in swabs for testing. They also asked them to reach out and get tested if they had symptoms. So all tests carried out are a combination of:
1. asymptomatic testing
2. testing sought due to symptoms

When we break down efficacy by these categories, the trial reports an efficacy of 76% in preventing *disease* (or symptomatic infection) and ~16% efficacy (with a lot of uncertainty due to small numbers) for asymptomatic infection. The combined efficacy for PCR +ves is 67%

Does this mean infection or transmission is reduced by 67%. It's difficult to know. Certainly the paper is careful not to make this claim. It only talks about reduction of PCR positives by 67% with the 'potential' to reduce transmission.

The Oxford press release also doesn't say this. It speaks about reduction in positive swabs with potential impacts on transmission. The estimate for reduction in infection will depend on other factors, including the compliance with routine swabbing. https://twitter.com/UniofOxford/status/1356656233642680321?s=20

This is hard to estimate from the paper. Of course impact on transmission is even harder to know. The media coverage of this is a bit concerning to me because I've been asked several times today whether it's ok for those vaccinated to socialise now that 'transmission is reduced'.

I think the media should ensure this is correctly and cautiously reported - it should not be reported as a 76% reduction in transmission. And those who are vaccinated should continue to take precautions to protect themselves and others.

Paper linked here:
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3777268