This nationwide register-based cohort study in Denmark examined whether grandchildren's birthdays (ages 1–16) were associated with increased SARS-CoV-2 infection risk in grandparents (n=1,106,493), using birthdays as a natural experiment from February 2020 to February 2022.
Grandparents had a 9.9% higher hazard of SARS-CoV-2 infection during the 7-day birthday risk window (days 2–8 post-birthday; 95% CI, 7.9%–12.0%), with greater risk for preschool-aged grandchildren (aHR 1.15; 95% CI, 1.12–1.18) than school-aged grandchildren (aHR 1.07; 95% CI, 1.04–1.09); no increased risk was seen during the Alpha-dominant period.
Observational design cannot fully rule out confounding; birthday celebrations were inferred, not directly observed; findings may not generalize beyond Denmark's testing infrastructure and cultural context.
Even small family gatherings like birthday parties carry measurable COVID-19 transmission risk to older adults, especially from younger (preschool-age) children. Clinicians advising high-risk older patients should flag culturally significant family events—not just large gatherings—as moments warranting extra precaution.
