Two recent studies have confirmed that two people previously infected with SARS-CoV-2, the virus that causes Covid-19, can be reinfected with the virus.
Interestingly, the two people had different outcomes. The person in Hong Kong showed no symptoms on the second infection, while the person from Reno, Nevada, had a more severe version of the disease the second time around. It is therefore unclear if an immune response to SARS-CoV-2 will protect against subsequent reinfection.
Does this mean a vaccine will also fail to protect against the virus? Certainly not. First, it is still unclear how common these reinfections are. More importantly, a fading immune response to natural infection,
Any infection initially activates a non-specific innate immune response, in which white blood cells trigger inflammation. This may be enough to clear the virus. But in more prolonged infections, the adaptive immune system is activated. Here, T and B cells recognise distinct structures (or antigens) derived from the virus. T cells can detect and kill infected cells, while B cells produce antibodies that neutralize the virus.
During a primary infection — that is, the first time a person is infected with a particular virus — this adaptive immune response is delayed. It takes a few days before immune cells that recognize the specific pathogen are activated and expanded to control the infection.
Some of these T and B cells, called memory cells, persist long after the infection is resolved. It is these memory cells that are crucial for long-term protection. In a subsequent infection by the same virus, the memory cells get activated rapidly and induce a robust and specific response to block the infection. A vaccine mimics this primary infection, providing antigens that prime the adaptive immune system and generating memory cells that can be activated rapidly in the event of a real infection. However, as the antigens in the vaccine are derived from weakened or noninfectious material from the virus, there is little risk of severe infection.