Table 4. Symptomatic seroprevalence by age groups and mask type, expressed in prevalence ratios.

All regressions include an indicator for each control-intervention pair. The regressions include controls for baseline rates of mask-wearing and baseline symptom rates. “Baseline symptom rate” is defined as the rate of surveyed individuals in a village who report symptoms coinciding with the WHO definition of a probable COVID-19 case. We assume that (i) all reported symptoms were acute onset, (ii) all people live or work in an area with a high risk of transmission of virus, and (iii) all people have been a contact of a probable or confirmed case of COVID-19 or are linked to a COVID-19 cluster. The analysis in the top panel uses the preregistered sample, equivalent to that in Table 2; it includes all people surveyed in the baseline household visits, excluding individuals for whom we did not collect midline or endline symptoms, symptomatic individuals from whom we did not collect blood, and individuals from whom we drew blood but did not test their blood. The analysis in the bottom panel replicates the regressions in the top panel but imputes the seropositivity of individuals from whom we did not draw blood. For symptomatic individuals from whom we did not draw blood, we simulate their symptomatic-seroprevalence status by using the average rate of conditional seropositivity among all symptomatic individuals. This analysis includes all people surveyed in the baseline household visits, excluding individuals for whom we did not collect midline or endline symptoms.

Parameter	All	<40 years old	40 to 49 years old	50 to 59 years old	≥60 years old
Preregistered sample: Drop individuals without blood draws
Intervention prevalence ratio for surgical mask villages	0.889** [0.780, 0.997]	0.967 [0.834, 1.100]	1.009 [0.817, 1.200]	0.772** [0.595, 0.949]	0.647*** [0.448, 0.845]
Intervention prevalence ratio for cloth mask villages	0.942 [0.781, 1.103]	1.058 [0.870, 1.247]	0.713** [0.459, 0.967]	0.838 [0.524, 1.153]	1.084 [0.769, 1.399]
Average symptomatic-seroprevalence in paired control villages†	0.0076	0.0055	0.0095	0.0108	0.0104
N individuals	287,349	146,306	35,839	24,086	27,943
N villages	538	480	384	348	360
Imputing symptomatic-seroprevalence for missing blood draws
Intervention prevalence ratio for surgical mask villages	0.873*** [0.801, 0.945]	0.917* [0.829, 1.005]	0.975 [0.862, 1.088]	0.815*** [0.688, 0.942]	0.701*** [0.577, 0.824]
Intervention prevalence ratio for cloth mask villages	0.890** [0.787, 0.993]	0.861*** [0.758, 0.965]	0.838** [0.678, 0.998]	1.153 [0.970, 1.336]	0.792** [0.601, 0.983]
Average symptomatic-seroprevalence in paired control villages†	0.0189	0.0152	0.0226	0.0229	0.0251
N individuals	321,383	177,708	51,676	37,340	43,431
N villages	570	566	528	504	534

***Significant at the 1% level.

**Significant at the 5% level.

*Significant at the 10% level.

†We report the mean rate of symptomatic seroprevalence at endline. This is not equivalent to the coefficient on the constant due to the inclusion of the pair indicators as controls.