There are fears that it may be possible to catch the coronavirus more than once, after a woman in her 40s in Osaka, Japan, tested positive for COVID-19 for a second time at the end of February. Studies and research are still going on to determine if this may be the case. So far, we do not have a definitive conclusion.

Before understanding whether or not the coronavirus can be caught twice, it is worth knowing how we build immunity to a virus in the first place.

When a pathogen (a foreign infection such as a virus or bacteria) enters the body, the immune system first has to recognise it as alien. There are specific blood cells whose job it is to patrol the body and quickly send out an alert if a new infection is encountered.

This alert stimulates the immune system to start producing very specific antibodies for the infection. This process takes time as a certain level of antibodies need to be produced to overcome the infection.

During this time the virus or bacteria is multiplying, and making you feel unwell as it does so.

It may take a few days or even weeks for your body to get to the level of antibodies needed to fight off the infection, during which time you have the symptoms of infection.

When enough antibodies are produced, the infection is overcome and you start to feel better.

The immune system then does something clever: it produces memory cells. These memory cells are ready and waiting should the same infection enter your body again in the future.

If that happens, they are immediately activated and work to destroy the infection before it has time to take hold and make you unwell again. The memory cells also send out a much quicker message to prompt your immune system into action.

In this case, your immune system works so quickly you do not feel unwell or even know you have been infected. This is because you now have immunity to that specific infection.

This immunity usually lasts for life - but not always.

More Doctor's Notes:

There are coronaviruses other than the one that causes COVID-19 disease. A lot of common cold viruses are from the coronavirus family and we do not gain lifelong immunity after being infected by them. People who have been infected with another type of coronavirus, severe acute respiratory syndrome (SARS), can have two to three years of immunity, so it really does vary.

Most experts believe that there are two ways out of this pandemic. We may find a vaccine against the virus, and that will allow rapid immunity to those who are administered the vaccine. Or enough people may get the virus through its current mode of spread and build up a natural immunity that will eventually result in "herd immunity" - where enough people are immune to it that it can no longer multiply and spread.

It is thought that roughly 60 percent to 70 percent of the population would have to be exposed to coronavirus in order to achieve this herd immunity.

What these solutions do not take into account is the prospect that the COVID-19 virus could mutate enough in the future to make it unrecognisable to a person's memory cells, or the prospect that some infections do not make memory cells that last for life.

So, is it possible to be infected with the COVID-19 virus more than once?

The first thing to remember is that we have only anecdotal reports concerning very small numbers of people. It is also not clear whether these cases involved a new infection or a relapse of the original infection.

A study done in China looked at whether monkeys who were infected with the coronavirus and developed antibodies to it could be reinfected when exposed a second time. Reassuringly, it found they were immune to it the second time around.

Immunologists agree that more research is needed. It is not clear if immunity from the COVID-19 virus will be lifelong, and data we do have on some other coronaviruses, such as the common cold, show that antibodies only give temporary immunity, usually lasting around three months.

However, some experts are optimistic. Martin Hibberd, a professor of emerging and infectious diseases at the London School of Hygiene & Tropical Medicine, says: "Although we need more evidence to be sure of this, people who have recovered are unlikely to be infected with SARS-CoV-2 again." This is another name for COVID-19.

The jury is still out on immunity: the initial reports look good, but there is much work to be done. The virus simply hasn't been around long enough for us to conclude that getting it will confer lifelong immunity.

The race is now on to watch and learn from those people who have recovered from the virus and to see whether the antibodies and immunity they have acquired last or not. Watch this space.