Waning immunity against symptomatic infection with Delta variant after 20 weeks, but not hospitalization or death for most groups

Reference

Andrews N, Tessier E, Stowe J, Gower C, Kirsebom F, Simmons R, Gallagher E, Thelwall S, Groves N, Dabrera G, Myers R, Campbell CNJ, Amirthalingam G, Edmunds M, Zambon M, Brown K, Hopkins S, Chand M, Ladhani SN, Ramsay M, Lopez Bernal J. Duration of Protection against Mild and Severe Disease by Covid-19 Vaccines. N Engl J Med. 2022 Jan 27;386(4):340-350.

Synopsis

Public Health England released a report based on national data about COVID-19 infection in vaccinated and unvaccinated persons. England has used the Pfizer and AstraZeneca (AZ) vaccines, with the first doses given more than 9 months ago. (The Moderna vaccine has also been used, but at the time of this study the data was not available beyond 10 weeks.) The authors of this report used a test-negative case-control design. Briefly, that means you identify persons who are symptomatic and test positive for COVID-19 and compare their rates of hospitalization and death with a group who are also symptomatic but test negative. Results were available from December 8, 2020, to August 20, 2021, and were stratified by the time from the second vaccination, with the longest-term group being greater than 140 days (20 weeks), and by whether infection was with the Alpha or Delta variant. For persons vaccinated more than 20 weeks ago, vaccine efficacy against symptomatic disease decreased to approximately 50% with the AZ vaccine and 70% with the Pfizer vaccine. However, vaccine efficacy against hospitalization for infection with the Delta variant was 78% for AZ and 94% for the Pfizer vaccine for those vaccinated more than 20 weeks ago, and vaccine efficacy against death was 93% for the Pfizer vaccine (adequate data for AZ were not available for mortality as an outcome). Results were similar when stratified by age, although confidence intervals are broad for the AZ vaccine. There was no difference in efficacy against hospitalization for those aged 40 to 64 years with or without risk factors for severe disease for the Pfizer vaccine, with some waning of efficacy for the AZ vaccine after 15 to 20 weeks in this age group. Among patients 65 years or older, there was greater waning of efficacy for those in the “clinically extremely vulnerable” group for both vaccines. Vaccine efficacy against hospitalization also waned significantly (to just under 70%) among persons 80 years and older. A caveat is that the oldest patients had a 3-week interval between doses, whereas younger patients generally had a dose interval of at least 12 weeks. The longer interval may provide better immunity, so the vaccine efficacy seen here may be somewhat better than that in the United States where a 3- to 4-week interval was recommended, but this may also be confounded by age differences.