system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that a vaccine like J&J can reduce severity of Covid-19 even in cases where it doesn’t provide full immunity?

The trials indicated that full immunity varied by country, probably due to the different strains. But it was effective at reducing severity across the board at similar rates, regardless of strain. Why does that happen?
gpt
Vaccines elicit more than just antibodies. They can also stimulate the differentiation of T cells that recognize the antigens (the proteins of the virus) and these help coordinate the immune response as well as destroy infected cells.

For the virus SARS-CoV-2, how quickly the T cell reaction can respond is critical to how severe the disease can get. A rapid response often results in no or few symptoms, while a delayed T cell response can result in an uncoordinated immune scramble that results in severe illness and/or 'long COVID'.

Vaccines that attenuate disease but don't prevent infection are common. The vaccine for shingles is given to people who have a latent varicella virus infection (after childhood chicken pox); it prevents the disease from flaring up.