Discussion
infection
INFECTION, RECRUITMENT
In this study, we evaluated the response rate to either postcard or letter invitations and reminders to participate in CA-FACTS, a population-based seroepidemiologic study to determine the burden of SARS-CoV2 infection in two Bay Area Counties in California. Our household response rates with two mailings, an initial one followed by a reminder two weeks later, was dependent on mailing type combinations: households receiving both a letter invitation and a letter reminder were 71% more likely than those receiving both a postcard invitation and reminder to enroll in the study – 13% vs. 8%—making two letter invitations a more effective recruitment tool than two postcards. This effect of mailing type on response rate was the same for both Santa Clara and Solano counties despite their very different population characteristics and response rates, confirming that letters were more appealing than postcards across the board.Reminders are important: paper invitations with paper reminders have been shown to improve response rates over a single mailing [Our finding updates two large studies: one of almost 14,000 households from 2005 showing that the completion rate of a telephone survey with advance letters was greater than with advance postcards, and another of 17,808 households from 2022 showing that the completion rate of a web-based survey was greater with advance letters with or without incentives than with postcards with or without incentives; additionally, both studies showed that letters were more cost-effective than postcards [Despite the positive effect of letters, we wish to emphasize that two postcards, one initial and one reminder, can be used as an effective recruitment tool, as their use more than doubled the response rate over an initial postcard alone. It is important to note, however, that in some cases postcards should not be used for recruitment, such as when the invitation reveals an element of health status, or when the sample is identified from a patient population frame [Limitations to our study include the biased respondents despite our representative sample, as respondents were more likely to be well educated, older and more White than present in the counties’ populations overall. This response bias is well known especially for racial groups [In summary, letter invitations with letter reminders are considerably more effective than postcards in recruiting population-based samples and should be used as essential seroepidemiologic studies are implemented to understand population immunity against epidemic agents and to prevent or decrease infection.