The association between SARS-CoV-2 seroprevalence and cross-border mobility for visiting family or friends among Dutch residents of a Euregional province

Background Border measures were implemented in many countries as infection prevention measures to interrupt between-country COVID-19 transmission. Border closings impact border region residents, as their professional and social lives are often intertwined across national borders. We studied whether crossing borders to visit family/friends in neighbouring countries (cross-border mobility) was associated with SARS-CoV-2 seroprevalence in Dutch Euregional residents. Methods SARS-CoV-2 serostatus (negative/positive) was assessed (pre-vaccination) using laboratory testing to determine previous infection. Visiting Belgian or German family/friends in February-March 2020 was questioned. The association between cross-border mobility and seroprevalence was tested using logistic regression analysis, adjusted for previously identified exposure factors. Results In 9,996 participants, 36.8% (n=3,677) reported cross-border family/friends. Of these, one-third (n=1,306) visited their cross-border family/friends in February-March 2020. Multivariable analyses revealed no positive association between cross-border mobility and seropositivity, for both participants living in a border municipality (ORfamily/friends not visited=0.90 [95% CI:0.78-1.04], ORfamily/friends visited=0.88 [95% CI:0.73-1.05]), and for participants not living in a border municipality (ORfamily/friends not visited=0.91 [95% CI:0.72-1.16], ORfamily/friends visited=0.62 [95% CI:0.41-0.94]). Conclusions This study provided no evidence of cross-border mobility as an important exposure factor for SARS-CoV-2. The results of our unique real-world study suggest that cross-border mobility did not substantially contribute to cross-border SARS-CoV-2 transmission in the Netherlands.