High Infection Attack Rate after SARS-CoV-2 Delta Surge, Chattogram, Bangladesh

To the Editor: After an initial serosurvey (1) to understand the prevalence of total antibodies to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in residents of the Sitakunda subdistrict was completed, a large epidemic wave hit the area, and nearly all publicly available samples genotyped via GISAID (https://www.gisaid.org) were the SARS-CoV-2 Delta variant (2,3). Of the total confirmed infections during the entire pandemic from the Chattogram District, 48.4% (48,253) were reported June 14–August 31, 2021. During September 21–October 9, 2021, we revisited all enrolled households and collected blood from 84% (1,938/2,307) of those tested in our initial serosurvey (Appendix Figure).

We tested 721 of the initially seronegative participants who agreed to a second blood draw using the same Wantai total Ab receptor-binding domain assay and found that 68% (492/721) had seroconverted in the approximately 3-month period between survey rounds (Appendix Table 1). Participation in the second round was not associated with serostatus in the first round. Among seropositive participants, 87 (18%) had received >1 dose of SARS-CoV-2 vaccine, and 28.3% (140/492) of those who seroconverted reported having had a sudden onset of >1 coronavirus disease–related symptom since the first serosurvey. Assuming no seroreversion between rounds, 88.2% (1,709/1,938) of participants providing blood in both rounds were seropositive by the second serosurvey. Using our previous methods (1), we estimated an adjusted seroprevalence after the Delta wave of 88.2% (95% CrI 85.4%–90.8%) for all participants and 87.9% (95% CrI 85.2%–90.6%) when including only unvaccinated participants (Appendix Table 2). Seroprevalence among children 1–9 years of age remained significantly lower when compared with 25–34 year olds (28% reduced risk for 1–4 and 16% for 5–9 year age groups; p<0.00001), unlike other age groups (Appendix Table 2). Mirroring evidence from around the world, the Delta variant led to a significant increase in SARS-CoV-2 transmission in Bangladesh, leaving the vast majority of people with detectable serum antibodies.

Appendix

Additional information on serosurvey of SARS-CoV-2 in Chattogram, Bangladesh