Editor's note: Updated on Aug. 31. The study cited in the article compares Covid Delta/Alpha viral loads in vaccinated individuals only, not the unvaccinated as was initially reported.] Scientists have discovered that fully vaccinated individuals carry 251 times the viral load of the Covid Delta strain in their nostrils as compared to the Alpha strain.

Many vaccines have heterogeneous effects across individuals. Additionally, some vaccines do not prevent infection, but reduce disease-associated mortality and transmission. Both of these factors will alter selection pressures on pathogens and thus shape the evolution of pathogen virulence. We use a ...