Fig. 3. Understanding protection from severe SARS-CoV-2 infection.

Vaccination provides greater protection from severe infection with SARS-CoV-2 than from acquisition of mild or asymptomatic infection. a | Studies show that breakthrough infection in vaccinees (with the Alpha variant or the Delta variant) leads to similar viral RNA levels at presentation or the peak to the levels seen in unvaccinated individuals. However, more rapid viral clearance is observed after the peak in vaccinated individuals. Data extracted from refs^55,56. Recall of neutralizing antibody responses (part b) (data from refs^81,82), CD4⁺ T cell responses (part c) (data from ref.⁸¹) and CD8⁺ T cell responses (part d) (data from ref.⁴⁴) occurs rapidly after vaccination of previously infected individuals. AU, arbitrary units.