Does COVID-19 Vaccination Protect Contact Persons? A Systematic Review

Abstract

The protective effect of COVID-19 vaccination for contact persons is controversial. Therefore, the aim of this review was to determine whether COVID-19 vaccination provides significant protection for them. A PubMed search was carried out using the terms “unvaccinated vaccinated covid” in combination with “viral load” and “transmission”. Studies were included if they reported original comparative data on the SARS-CoV-2 viral load, duration of SARS-CoV-2 detection, or SARS-CoV-2 transmission rates. A total of 332 articles were identified, of which 68 were included and analyzed. The differences in the viral load were equivocal in 57% of the 35 studies, significantly lower in the vaccinated in 11 studies and in the unvaccinated in 3 studies. The infectious virus levels were significantly lower in the vaccinated in two out of six studies. Virus clearance was significantly faster in vaccinated subjects in two of eight studies (detection of viral RNA) and two of four studies (detection of infectious virus). The secondary attack rates were significantly lower in vaccinated index cases in 6 of 15 studies. The vaccination status of contacts was described in two of the six studies and was 31.8% and 39.9% lower in households with an unvaccinated index case. The inconsistent and variable differences in the viral load, viral clearance and secondary attack rates between vaccinated and unvaccinated individuals, especially during the omicron predominance, suggests that COVID-19 vaccination is unlikely to prevent a relevant proportion of transmissions to contact persons, taking into account the relevance of the immunological status of the contact population (vaccination rates and previous infection).