. >> reporter: martin hirsch of massachusetts general hospital agrees. he says genetics and several other factors affect immune response. >> younger people, healthy people without any underlying conditions are the most likely to develop prolonged antibodies. >> reporter: with covid-19, the world health organization says we need more evidence about how effective the antibodies are and how long immunity will last. >> when we're talking about this idea of potentially getting infected for a second time, what time frame would sound reasonable to you for that to make sense? >> i think those people who test positive then negative and then positive within a few days or a few weeks are almost certainly not re-infected. i think if you go beyond that, a few months, a few years, then you can start talking about re-infection. >> reporter: antibody is not synonymous with immunity. antibody just tells you you've been infected before. immunity is more this complex idea. it's a continuum. and it depends on several things. it depends on the number of antibodies you produc

. >> reporter: martin hirsch of massachusetts general hospital agrees.l other factors affect immune response. >> younger people, healthy people without any underlying conditions are the most likely to develop prolonged antibodies. >> reporter: with covid-19, the world health organization says we need more evidence about how effective the antibodies are and how long immunity will last. >> when we're talking about this idea of potentially getting infected for a second time, what time frame would sound reasonable to you for that to make sense? >> i think those people who test positive then negative and then days or a few weeks are almost certainly not reinfected. i think if you go beyond that, if you look at a few months, a few years, then you can start talking about reinfection. >> reporter: >> lisa's rate, tara. lisa's right when she says it is confusing. help us understand. how could it be that you can have the antibodies and not be immune to the coronavirus? >> reporter: it's definitely confusing. a lot of people equate the two terms. antibody is not sy