Can you get it again after you’ve had it?
Knowing whether or not one infection gives us immunity would dramatically affect how we decide to deal with the coronavirus pandemic (herd immunity vs containment), which diagnostic tests we push for (such as antibody tests to show who's been infected in the past) and how we develop a vaccine. So basically, that’s a very good question, and unfortunately we don’t yet have a definitive answer.
When our bodies get infected with something (be it bacteria/virus/etc), it mounts an immune response to fight the infection. One part of the immune response is the production of antibodies, which circulate in your bloodstream and stick specifically to the offending intruder. Some of these antibodies hang around in the body after infection and mean the second infection is fought much faster or avoided all together. This immunity forms the basis of vaccines (check out this post and scroll down to find out how vaccines work) and herd immunity.
Some news stories have reported cases of second infections, including a man onboard the Diamond Princess ship and a woman in Japan, as well as cases in China. There is speculation that these are cases of “relapse” rather than “reinfection”. This can result from either inaccurate testing or the body not developing a proper immune response, which causes the virus to become dormant instead of dying. Relapse like this has been described for SARS, a close relative of SARS-CoV-2 that causes COVID-19.
More controlled experiments can be done with animals. An experiment using macaques found that after infection with SARS-CoV-2, the animals develop antibodies and are immune to second infection 2 weeks later. These experiments are much more controlled than real-life cases, and it’s possible to measure the exact amount of virus and antibodies in the animal from day 1, whilst for people that become infected tests are only run once they show symptoms. On the other hand, further data is needed to be able to relate this data to humans - and perhaps more importantly - to understand if this immunity is life-long.
Other viruses from the same coronavirus family as SARS-CoV-2 cause common colds. We do develop immunity against these viruses after infection, but it’s not life-long. In this study humans were infected with a coronavirus that causes the common cold called 229E and their antibodies measured. The results showed that they built an antibody response a week after infection, but a week later these antibodies slowly reduced in levels. There were still some antibodies left a year later, but they did not always confer immunity. SARS-CoV is a closely related virus to SARS-CoV-2 which caused the SARS epidemic in 2002. A 3-year follow up study was done to measure antibodies in patients who had contracted SARS, and found that antibody levels drop off after around 3 years, meaning their immunity might have been lost.
We know that our bodies make antibodies against the SARS-CoV-2 virus after infection (some diagnostic tests are completely based on these antibodies). We also know that antibodies are able to confer immunity against many diseases at a high enough concentration. What we don’t know yet for COVID-19 is how long our antibodies hang around for. Although current data based on animal experiments and related viruses suggest that we do build immunity, long-term studies are needed to see how long this immunity will last.