Thread by @gbosslet

Headlines about a recent study point out that a recent study of the Astra Zeneca vaccine appears to slow transmission of the SARS-CoV-2 virus.

nyti.ms/39HJ78f

I think it is important to show what this study does and does not imply.

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You can find the manuscript here: bit.ly/3cF1Fbe

NOTE THAT THIS IS A PREPRINT AND HAS NOT YET UNDERGONE PEER REVIEW.

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Vaccines are good at reducing symptomatic #COVID19.

What people really want to know is if they can take off their masks once they have the vaccine. After I am vaccinated, if I don’t have symptoms, could I be infectious?

I will refer to this as slowing asymptomatic spread.

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Slowing asymptomatic spread is difficult to prove without large epidemiologic studies.

The new AZ study tried to get at this by asking the participants in the UK arm to do weekly self-swabs of their nose and throat to see how many had positive swabs without symptoms.

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It is correct to conclude that the AZ vaccine slowed spread. This was almost completely driven by spread of symptomatic disease.

It did NOT prevent asymptomatic positives after a single dose. It did to a small extent after the 2nd dose (but only with the low-dose regimen).

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There were 130 asymptomatic SARS-CoV-2 PCR swabs overall in the trial- 73 in the control arm, and 57 in the vaccine arm.

We don’t know if these asymptomatic positives would be infectious (it would be nice to know the cycle threshold for these).

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This vaccine is definitely effective.

And the rate of asymptomatic PCR positivity was low (130/8207= 1.58% overall), which is good.

But I don't think we should conclude that we can loosen public health measures until everyone has a chance to be vaccinated.

#MaskUp
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Would love to hear what others think about this. I was struck by the headlines and really worry it was a bit hyperbolic, but maybe that was just my own neurosis.

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