Transmissibility of SARS-CoV-2 among fully vaccinated individuals

With interest we read the paper by Anika Singanayagam and colleagues¹ assessing the secondary attack rate (SAR) of SARS-CoV-2 in 204 vaccinated and unvaccinated household contacts exposed to 138 vaccinated and unvaccinated index cases. Here, we want to point out the importance of adjusting for age when comparing vaccinated and unvaccinated individuals.

The authors report a similar SAR among household contacts exposed to fully vaccinated and unvaccinated index cases (25% and 23%). Although not explicitly stated by the authors, this finding hints towards no effect of vaccination on transmission and was reported as such by the media in the UK and the Netherlands—and possibly other countries.2, 3 However, age is a confounding factor in this observation if age is associated with both vaccination status and the risk of transmitting SARS-CoV-2. Indeed, the study indicates a higher peak viral load with increasing age, consistent with lower infectiousness in children. In addition, although the age distribution of all included index cases and contacts is not presented, table S2 in the appendix to the Article provides data for a subset of participants testing positive for SARS-CoV-2, showing that a large proportion (78%) of unvaccinated participants were younger than 18 years, whereas none of the vaccinated participants were. These findings together suggest that the infectiousness of the included unvaccinated index cases was lower than that of the included vaccinated participants because of younger age. Therefore, the presumed lack of vaccine effect on transmission might be largely due to confounding by age, which the authors did not address. In our analysis of vaccine effectiveness against transmission in the Netherlands, adjustment for age of index cases and contacts indeed had a large effect on vaccine effectiveness estimates.⁴ Therefore, vaccine effectiveness against transmission reported by Singanayagam and colleagues is probably an underestimate.

Also, the reported vaccine effectiveness against SARS-CoV-2 infection (34%) is likely confounded by age, as vaccination status is associated with age, and younger age is associated with reduced susceptibility to acquiring SARS-CoV-2 infection.⁵

In these times, when evidence-based confidence in vaccines is crucial to reduce the impact of the COVID-19 pandemic on mortality and morbidity, data on effects of vaccination should be adequately and unambiguously reported by the scientific community in order to avoid misinterpretation of the data by the public and the media.

We declare no competing interests.