This just shows how important the first step of immunization is. Otherwise, lymphocytes would have no way to know if a first time virus was present or not. The immune response would be much slower and the virus could multiply without any initial barrier. Depending on the type of virus, the symptoms could be devastating, but many times avoidable. If taken care of.

An interesting strategy using this natural mechanism, without previous vaccination, is used for viruses with a very slow multiplication rate. Therefore, with large incubation periods.
See the case of the rabies virus. It is usual to vaccinate someone right after getting bitten by a possibly infected animal. The vaccine will trigger a fast response from our immune system (a lot of antigens circulating). The adaptive immune system produces the newly rabies virus targeted lymphocytes at a much faster rate than the virus can multiply. Now, the new lymphocytes are able to detect the cells infected with rabies viruses before symptoms could appear.

Thanks for sharing!