Impact of diagnostic delay in COVID-19 index cases on SARS-CoV-2 transmission to household contacts

Introduction Diagnostic delay in COVID-19 index cases may increase household transmission of SARS-CoV-2. A prospective epidemiological study was conducted to analyze SARS-CoV-2 infection among household contacts of confirmed COVID-19 cases. Methods Recruitment of index cases and their household contacts took place in eight primary care centers. This study included 227 index cases and 329 household contacts and examined the association between the time from symptom onset to index case diagnosis and the household transmission rate of SARS-CoV-2. Results The prevalence of infection among contacts was 38.9%, increasing with age and reaching 60.6% among individuals aged 65 years or older. Diagnostic delays longer than two days were associated with a higher risk of infection: contacts exposed to index cases diagnosed with a 3–4-day delay had twice the risk compared with those diagnosed earlier (adjusted odds ratio, aOR = 2.1; 95% CI: 1.1-3.7). Older age was confirmed as a risk factor (aOR = 6.0; 95% CI: 2.3–15.7), while previous SARS-CoV-2 infection and vaccination of the index case were identified as protective factors (aOR = 0.5; 95% CI: 0.3–0.9 and 0.3; 95% CI: 0.1–0.6, respectively). Conclusion These findings demonstrate that diagnostic delay significantly increases household virus transmission, particularly among older adults, highlighting the need to strengthen early detection mechanisms and ensure rapid access to diagnostic testing in all settings.