Introduction
infections
RECRUITMENT, DISEASE, INFECTIONS
Reported cases of a disease with epidemic potential can represent a small, biased fraction of all infections, as illustrated by the current pandemic of COVID-19 and SARS-CoV2 [Representative samples of populations are best identified via physical mailing addresses; in the US, address-based samples (ABSs) generated from the US Postal Service Delivery Sequence File—which is a list of all addresses to which mail is delivered by the USPS—are based on nearly all US households (90–98%) [Over the first two years of the COVID-19 pandemic, the public was aware of and generally interested in SARS-CoV2, and, given that testing kits were essentially unavailable at that time [To recruit participants, we sent an invitation to a population-based sample of households. The invitation stated that up to three members of the household could receive free serologic testing for SARS-CoV2 by collecting a finger stick blood spot at their home, completing a questionnaire and returning the blood spot card by mail; the recipient accepted participation either online or by telephone. With the goal of enhancing recruitment, we assessed whether type of mailing—postcard or letter—affected response rates. For our first serologic study, we contacted a random selection of 10,000 households in both Santa Clara and Solano counties and evaluated differential response rates to postcards and letters.