Ralph Baric: "I saw some very interesting data from Stan Perlman the other day, who has been looking at serum neutralization titers of MERS patients from the Middle East kingdom of Saudi Arabia area and it's quite intersting that people peak fairly quickly with high neutralization titers but then they wane over the next year to almost background levels or just slightly above background levels by the second year, and with MERS there have been several reports of people who have seroconverted. They were rtPCR positive and their serum neutralizing titers and even ELISA titers went to almost zero within a few months."
Baric: "And it has not been studied and it should be studied, and this is the contemporary human Coronaviruses -- nobody knows how they maintain themselves in human populations. They don't undergo rapid antigenic variation like influenza. There's not 115 common cold or corona virus type genotypes or whatever they're called, serotypes. Sorry Vincent, I just butchered the coronaviruses."
Vincent Racaniello: "That's ok." <laughter>
Baric: "So one hypothesis is that they cause a transient protective immune response that wanes quickly and then they can reinfect and cause mild upper-respiratory tract infections and that's how they maintain themselves. So it is quite possible.. there's been a number now of reported cases in China of SARS2 infections where people were documented to be infected and recovered. They were rtPCR negative. They went home and they became reinfected a month later or so."
Baric: "In this case the United States has sufficient cases that we can actually track the serologic responses of the individuals and their general immune.. both B- and T-cell responses after infection and we can get a handle on the long term immunity that may be elicited after infection."
[1] - http://www.microbe.tv/twiv/twiv-591/
[2] - about 15 minutes and 50 seconds in to the program
In the first paragraph he speaks of evidence that immunity waned over time in the case of a closely related disease called MERS.
In the second paragraph he talks about the mystery of how all human coronaviruses (including some of the viruses that cause the "common cold", the virus that causes MERS, and SARS-Cov-2: the virus that causes COVID-19, aka "The Coronavirus") keep themselves alive in the human population. He didn't say so explicitly, but the implication is that we should expect these coronaviruses to die out as humans get sick, recover, and acquire immunity to them.. but that's not what happens. People get sick with these same viruses again and again and again.
In the case of the flu (which is not a coronavirus), the way it keeps reinfecting people is that it mutates a lot in ways that will evade acquired immunity, so any immunity a person had from having had and recovered from the flu will not keep them from getting sick with another, mutated variation of the flu.
But that doesn't happen with human coronaviruses. People keep getting infected with the same strain. So the educated guess (the hypothesis) is that being infected and recovering from the coronavirus does grant some immunity, but that immunity quickly fades and then people get reinfected with the same strain because they no longer have that immunity (or have too little of it to keep from getting infected).
Then he mentions reports from China of people getting infected with the coronavirus, recovering, and then getting reinfected (which should support the above hypothesis, but we can't be sure because China has not been actually monitoring the levels of antibodies in infected and recovered people in their population).
In the final paragraph, Baric says he expects the US health care system to closely track the level of antibodies in infected and recovered people over time, which should give us some insight in to whether this hypothetical reduction in immune response really does happen.
If you’re interested, I can post some news reports. Forbes actually has a pretty good rundown of the various news articles, but Forbes isn’t always the best place for facts or safe browsing, so I’ll avoid posting that one. In the interest of fairness, I can also post (much older) news reports that called HIV/AIDS “Gay Related Immuno-deficiency” and we know how good that science was.
Additionally, people who are asymptomatic can test negative on all of their tests and only be diagnosed via other means. See Figure 1 here: https://link.springer.com/content/pdf/10.1007/s11427-020-166...
