Mark Thompson of the CDC COVID-19 Response Team and colleagues also demonstrated that the vaccines were highly efficient at lowering the viral load, febrile symptoms, and the duration of illness in people who acquired breakthrough infections despite being vaccinated. Full immunization was found to be 91% effective in preventing SARS-CoV-2 infection in a large prospective study of approximately 4,000 people, whereas partial vaccination was found to be 81 percent effective. According to the researchers, if more evidence shows that these vaccines reduce viral load and, as a result, limit SARS-CoV-2 infectivity, it could mean that the vaccinations are not only efficient in preventing infection but also in reducing the effects of breakthrough infections. The researchers compared the viral RNA burden of vaccinations that had a breakthrough infection to unvaccinated infected individuals to determine the efficiency of partial and complete vaccination in avoiding SARS-CoV-2 infection. Vaccinated individuals got either the BNT162b2 vaccine from Pfizer-BioNTech or the mRNA-1273 vaccine from Moderna, and efficacy was estimated after adjusting for geography, occupation, and local virus circulation. The vaccine’s effectiveness in preventing SARS-CoV-2 infection was estimated to be 91 percent for complete vaccination and 81 percent for partial vaccination. Following at least partial immunization, an adjusted analysis revealed a 40.2 percent decreased viral RNA load.

Reference

New data from CDC on effectiveness of Pfizer and Moderna COVID-19 vaccines, https://www.news-medical.net/news/20210606/New-data-from-CDC-on-effectiveness-of-Pfizer-and-Moderna-COVID-19-vaccines.aspx