Figure. Spike antibody responses by surveillance and population survey sampling methods for study of acute febrile illness surveillance similar to household serosurvey for estimating population immunity, Dominican Republic. The study compared SARS-CoV-2 spike antibody data collected during July–October 2021 in the same provinces from a longitudinal AFI surveillance system embedded in routine healthcare settings (“surveillance”) (4), which included routine blood collection for serologic testing; and a multistage, population-representative household serologic survey (“survey”) (1). Participants were matched by age and number of COVID-19 vaccine doses at a 1:5 ratio (surveillance, n = 115; survey, n = 575). A) Histogram showing number of participants by sampling date. B) Density ridge plots illustrating titer distributions by sampling method. Dashed gray lines indicate previously reported spike antibody thresholds associated with >75% protection against symptomatic infection for Mu (10^1.23), Delta (10^1.88), BA.1 (10^2.80), and BA.4/5 (10^3.06). Threshold for XBB.1 was inferred based on ≈10-fold lower neutralizing response relative to BA.4/5. C) Dot-whisker plots showing estimated proportion of participants with antibody levels corresponding to >75% protection by variant or subvariant (underlying data in Appendix Table 1). Dots indicate point estimates; whiskers indicate 95% CIs. Protection thresholds taken from previously published variant-specific correlates of protection (5). Estimates show percentages of persons above those thresholds (uncertainty in thresholds [reported 95% CIs] not propagated into percentages).