StarAC17 wrote:In theory when you contract the same infection again doesn't the replication of the initial virus have to happen to an extent to alert the immune system of the presence of the virus and then the appropriate antibodies are released to neutralize the virus without the host getting sick again. This can explain why someone could test positive for Covid-19 again but not necessarily going to get sick again. Especially given the long incubation time of covid19.
Is this correct or is the immune system respond even faster and if it does, how? Also can an immune person be an asymptomatic carrier even though they will not get sick for however long the immunity lasts.
Interesting question. So here is a study looking at this question in a different coronavirus called 229E. In this study, they infected volunteers with HCoV-229E and monitored their antibody responses. They found that there was an initial antibody response that faded away by 6-12 months. 12 months later, they rechallenged the volunteers with the same virus and found that all of them became infected again, which was defined by recovery of infectious virus by nasal irrigation on at least one day after the challenge, but only one of them got sick (and only mildly at that). They also found that the duration of the infection was a lot shorter the second time around. So yes, if there is only cell-mediated immunity around in the form of memory T cells, the virus will start to replicate, but it won't get very far before the T cells suddenly come screaming down on it like a pack of wolves and snuff it out. Often, this happens so quickly that the patient doesn't even get sick.
First of all, COVID-19 often makes people much sicker than the other four endemic HCoVs. So we can probably expect that for those who do recover, the immune response will be more robust because that illness is fundamentally caused by an inflammatory response, so more inflammation=more robust immune response (without putting too fine a point on it...there are eleventy caveats to that statement). So my guess is that immunity for people who survived COVID-19 will probably last longer, especially for those who got pretty sick. But for most viruses, symptoms do help with transmission. They may not be *strictly necessary* for transmission, but those symptoms help to increase R0 (R-naught, the average number of people who a sick person will infect). So even if people ultimately *do* get reinfected with SARS-CoV-2, I would expect that infection to be asymptomatic or very mild
And that's a big issue that a lot of people seem to have missed. "Contagious" isn't just a yes/no thing. It's a likelihood. *CAN* someone with no symptoms transmit it? Yes. Are they as good at transmitting it as someone who is coughing and sneezing? Probably not.