To understand the full impact on vaccines on disease spread, we need to know if they prevent transmission as well as stopping vaccinated individuals from getting sick with COVID. This new preprint from Public Health England on over 550,000 households provides further evidence that the Pfizer/BioNTech and Oxford/AstraZeneca COVID vaccines are effective in reducing transmission of the virus between individuals as well as preventing people getting very ill with disease. This is very promising.

While this study brings welcome news, we must not be complacent. There is still much we need to learn about how COVID vaccines affect transmission. It is still very important for us all to get two doses of the COVID vaccine to ensure we receive the optimal and longest lasting protection, both for ourselves and our communities.

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Does the press release accurately reflect the science?

Yes, the press release is an accurate summary.

Is this good quality research?  Are the conclusions backed up by solid data?

Yes – “There were 552,984 residential households of two to 10 people where there was at least one case.” After excluding households that didn’t meet the (very appropriate) study criteria there were 365,447 households, 1,018,842, and 102,662 secondary cases in the study. These are substantial numbers, so the conclusions are robust.

How does this work fit with the existing evidence?

We have mounting evidence of the real-world effectiveness of the Covid-19 vaccines. Vaccines are more effective against  more serious illness. They are more effective against death and critical care admission, and against hospital admission, than at preventing symptomatic disease that does not require hospital admission. They seem to be still less effective against asymptomatic disease – although this is harder to evaluate without routine testing of asymptomatic people. And their efficacy in terms of preventing people from being infectious is harder to evaluate – that is where this study comes in.

Have the authors accounted for confounders?  Are there important limitations to be aware of?

The authors have been very cautious. They may, for example (as they describe in the discussion) have underestimated the effectiveness of the vaccines in preventing transmission, as their definition of secondary cases is likely to have included some co-cases (people who acquired their infection from the same source as the “index case”, rather than from the index case).

What are the implications in the real world?  Is there any overspeculation?

These findings are really important. They add to our reasons to hope that the vaccines will truly add to herd immunity. The evidence was already mounting that vaccination will prevent people from becoming infected (and if they aren’t infected, they can’t transmit the infection). This study shows that even if people who are vaccinated do become infected, they are considerably less likely to be infectious, and to pass the infection on to others.

Any other comments?

This is an extremely encouraging set of findings.

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