Here's what Ralph Baric, an epidemiologist from the University of North Carolina Chapel Hill, had this to say on episode 591 of This Week in Virology[1] a few days ago:

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 RT-PCR 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 RT-PCR 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.

On the other hand, from a letter read on an earlier TWiV[2]:

I'm an infectious disease doctor and would like to point out an important factor in diagnosing this infection (and all respiratory pathogens that use nucleic acid probes for specimen collection). This is of particular concern now that China has transitioned to a much more problematic clinical diagnosis based more on clinical symptoms than PCR testing.

While it may seem trivial how a health care worker jams a swab in to someone's throat or nose, technique is important. If anterior naries (front of nose) swabs or side of mouth swabs are done, rather than the true posterior nasopharynx or oropharangeal swabs, the sensitivity drops dramatically.

We have shown this to be true repeatedly with diagnosis of influenza and respiratory pathogens, and I am disappointed that it has not been mentioned more in discussions about the PCR tests, missing the diagnosis in people who are positive and negative then positive again.

We often see people admitted from the ER with the perhaps carelessly collected flu test who miraculously are flu positive by the time they are admitted. Like so many cultures and diagnostics in infectious diseases, specimen collection and handling is critical.

I warn my patients about the unpleasantness of a nasopharangeal swab and jokingly tell them that if it doesn't cause a wincy-face reaction we haven't collected specimens from where the viruses reside.

[1] - http://www.microbe.tv/twiv/twiv-591/ about 15 minutes and 50 seconds in to the program

[2] - http://www.microbe.tv/twiv/twiv-588/ at about 1 hour and 38 minutes in