A new study from Public Health England (PHE) and Cambridge University was published in the Lancet last week, claiming to find that the Delta variant “doubles a patients’ risk of hospitalisation compared to the Alpha variant”, as the Telegraph reported (as did the BBC).
Reviewing over 40,000 sequenced positive PCR test results from England between March 29th and May 23rd, 2021, the researchers found that 2.2% of Alpha infections (764/34,656) and 2.3% of Delta infections (196/8,682) were hospitalised within 14 days of their first positive test. However, once they adjusted for factors such as age, ethnicity and vaccination status they found the risk of hospitalisation from Delta more than doubled compared with Alpha (a 2.26-fold increase).
The authors took the opportunity to use the results to stress the importance of being vaccinated, noting that only two per cent of those hospitalised were double vaccinated. However, this data is out of date, as more recent data from the main Delta surge suggests that vaccine efficacy against Delta infection may be as low as 15%.
The finding that Delta is more than twice as serious as Alpha is surprising as official data shows that hospitalisations have been much lower with Delta than Alpha. Data in the PHE technical briefings on the variants of concern shows that between February 1st and August 15th, 2.9% of sequenced Alpha infections resulted in an overnight hospital stay, compared with 1.9% of sequenced Delta infections. Technical Briefing 21 (the most recent) acknowledges this but adds that “a more detailed analysis indicates a significantly greater risk of hospitalisation among Delta cases compared to Alpha (see page 50 of Variant Technical Briefing 15)”.
Technical Briefing 15 quotes the findings, then in pre-print form, which are now published in the Lancet study (I criticised an earlier version of the claim here).
March 29th to May 23rd, the study period, is notable for being a period of low prevalence in England, when there were only around 2,000 reported infections a day (compared to a peak of around 50,000). In the over-60s in that period there were only 100-200 reported infections per day.