Researchers from Israel have reported that a newly approved mRNA vaccine, BNT162b2, not only does the vaccine reduce the incidence of disease by 95% in vaccinated individuals, at seven days from the completion of the vaccination, but it reduces the viral load fourfold in breakthrough infections.
The mechanisms by which such an outcome can be achieved include a reduction in the number of susceptible individuals, a reduction in the viral load, and thus, reduced viral shedding from people who develop the infection after vaccination. The result is thus, in all scenarios, a reduction in the number of new infections. The researchers examined the positive results of RT-PCR. Among the positive PCR tests, the cycle threshold (Ct) values over time reflected a substantial reduction in average viral load at 12 days post-vaccination.
Given that a Ct value difference of 1 corresponds to almost twofold higher viral particles per sample, the differences found in this study indicate that the viral load in infected vaccinated individuals is almost 3 to 4.7 times higher compared to that in vaccinated individuals who have a breakthrough infection within 12-28 days after vaccination. This means such patients would shed much lower levels of virus, be much less contagious, and would develop much milder disease. This is an important observation on vaccine impact, though the study design does not allow causality to be inferred.
Ref link: https://www.medrxiv.org/content/10.1101/2021.02.06.21251283v1