A new study has suggested that high viral loads in patients with post-vaccine breakthrough infections do not necessarily translate to increased contagiousness of SARS-CoV-2.

The study from The Netherlands, published as a preprint on medRxiv, included 24,706 healthcare workers, who had been vaccinated with either Moderna, Pfizer/BioNTech, Johnson & Johnson, or AstraZeneca COVID-19 vaccines. Of these, 161 patients developed breakthrough infections from April to July 2021, majority of which were delta variant infections (B.1.617.2).

Researchers estimated the viral loads in the vaccinated as well as the unvaccinated healthcare workers (infected with the D614G strain) with the help of RT-PCR and virus culture (as a marker for infectivity). The viral loads in the vaccinated study subjects were found to be high and comparable to the viral loads in those healthcare workers who had COVID-19 last year following infection with the original strain of the virus and were unvaccinated. The infectious virus was found in 68.6% of breakthrough infections and the Ct-values decreased throughout the first 3 days of illness

Nevertheless, it was found that compared to the unvaccinated healthcare workers, there was less chance of detection of the virus in nasopharyngeal samples of the vaccinated healthcare workers with breakthrough infection. The researchers hypothesised that this could be due to the neutralizing antibodies produced due to the vaccine.

Therefore, it was concluded that post-vaccine breakthrough infections do occur. But, despite the reduced viral shedding, as demonstrated in this study, breakthrough cases are still capable of spreading the infection. Adherence to covid-appropriate behavior is of paramount importance, even for the fully vaccinated persons.

(Source: Medscape; medRxiv, online August 21, 2021)