This thread pulls together some of the information underpinning the decision to delay the 2nd dose of COVID19 vaccine. The brief statement from the Joint Committee on Vaccination and Immunisation (JCVI) outlining the rationale can be found here 1/:
https://www.cas.mhra.gov.uk/ViewandAcknowledgment/ViewAttachment.aspx?Attachment_id=103741

Pfizer vaccine efficacy d15-21 is estimated at 89% (if include infections in 1st 14d, before you would expect the vaccine to take effect, it drops to 52%). Moderna vaccine was tested at 1 dose (996 vaccine, 1079 placebo). VE (d14 onwards) was 92%. Median follow up 28d (1 – 108).

AZ/Oxford single dose vaccine efficacy (from d22) estimated at 73%. There was high protection from hospitalisation suggesting high short-term protection from severe disease. 3/

Many people are concerned the vaccine may work less well if dose 2 is delayed. However the immune response provoked by dose 1 won’t disappear by 12wk. In fact, often a longer dosing interval gives stronger immunity. There are hints this may be the case for the AZ/Ox vaccine. 4/

One concern is whether “partial immunity” after 1 dose increases the chance of escape mutants emerging. This is discussed in more detail here (but it’s not clear that the risk is bigger than with natural infection) 5/ https://twitter.com/LucyWalkerlab/status/1353124575710769153

The risk of generating escape mutants was what initially concerned me most about the dose 2 delay. But, I’ve learnt from those working on virus evolution that we can’t consider this in the same way we consider antibiotic resistance (see thread cited in previous tweet). 6/

Early data from Israel suggest 1 dose of Pfizer vaccine may reduce the number of people testing + for virus. I think there are 2 datasets showing either a 33% (>60yr only) or 60% (all ages) drop in + tests at d14. 7/ https://twitter.com/beecellnumbers/status/1352908126626918403

The vaccine efficacies cited at the start of this thread reflect symptomatic disease – the new data suggest the vaccine may limit infections as well as disease (even after 1 dose). This could have important implications for reducing transmission. 8/

We clearly need more evidence to either confirm or refute assumptions being made on limited data. As vaccine availability increases, and more data arrives, the balance of judgement may change. My personal view is that the delay is reasonable given the circumstances. 9/

It’s worth noting that PHE has a surveillance system to collect data on people with a positive coronavirus test after receiving 1 or 2 vaccine doses: 10/ https://www.gov.uk/government/publications/covid-19-enhanced-surveillance-of-cases-in-vaccinated-individuals/reporting-to-the-enhanced-surveillance-of-covid-19-cases-in-vaccinated-individuals

There is also an excellent explainer from @wellcometrust on vaccine dosing schedules here. 11/ https://twitter.com/wellcometrust/status/1353374257615671296

Finally, note that even fully vaccinated people can carry virus. Protection is high but not 100%. A few will get #COVID19 and many more will likely carry virus without symptoms. So important to continue virus suppression measures for now. END