The most striking point from this week’s UKHSA Vaccine Surveillance report – with data for the four weeks ending January 9th – is the sharp decline in unadjusted vaccine effectiveness (calculated from raw data, so not adjusted for potential confounders such as risk factors and testing behaviour) against hospitalisation (see above). The decline is particularly sharp for under-50s, with 18-29 year-olds dropping to 38% (meaning the hospitalisation rate was 38% lower among the vaccinated (two or three doses) than the unvaccinated). The sharpness of the drops coincides with the age groups with the lowest third-dose coverage (see below), which suggests it may be a waning effect accelerated by Omicron and offset by boosters.
Another possible explanation is that it is an artefact of the high number of incidental hospital admissions in recent weeks, with more than half of Covid hospital admissions being treated primarily for something else. Given vaccines offer little to no protection (or worse) against infection, they will offer the same lack of protection against incidental hospital admission as Omicron spreads in hospitals or is found in those admitted for other reasons. The difference by age may arise because infection rates in those under 50 are currently much higher than in those over 50. In truth, it is likely to be a bit of both, but without data specifically on those being treated primarily for Covid it’s difficult to get to the bottom of which is making the biggest difference.
Unadjusted vaccine effectiveness against infection continued to decline in older age groups and in under-18s this week, presumably due to the waning of the third and second doses. It is negative in all over-18s, meaning the infection rate is higher in the vaccinated than the unvaccinated; in the 18-70s it is lower than minus-100%, meaning the vaccinated are more than twice as likely to have an infection. It did however rise slightly in the 18-50s this week, from a very low base. UKHSA continues to claim this extraordinary data is a result of confounders such as different risk factors and testing behaviour between vaccinated and unvaccinated. However, the agency still has not published any data on these confounders (e.g. testing behaviour and co-morbidities by vaccination status), despite being asked repeatedly, nor made any attempt to estimate adjusted vaccine effectiveness based on this data. If you would like to ask it to do this you can email its head Mary Ramsay here (Twitter here).