The evidence around COVID-19 and immunity is rapidly evolving. Based on what we know so far, COVID-19 patients become immune (protected from reinfection) for at least 3 months, if not longer, after they recover from the illness. However the virus is still new and there are no long-term studies published about it yet, so it's hard to accurately assess how long immunity persists in the population.

A study in Nature found that men's immune systems, particularly older men over the age of 60, may make them twice as likely to get severely sick or die in comparison to women from the same age group. These results are not entirely surprising. Scientists already know that women's immune systems typically are stronger at fighting other illnesses, compared to men. Public health experts believe this may mean that men need to rely more on vaccination, rather than natural infection, to safely protect against recurring infections.

Levels of antibodies, which are particles an infected person produces to fight off an illness, typically shrink after recovery. In August 2020, the New York Times reported the first documented case of re-infection with COVID-19. A 33-year-old man, who was first diagnosed with COVID-19 on March 26, 2020, had no detectable antibodies after his first infection. On August 15, 2020, he tested positive again and researchers confirmed that the test result was due to a new infection, rather than a prolonged previous infection.

Antibodies are only one component of the body's complex immune system and its response to COVID-19. Antibodies prevent a future infections, but other mechanisms, like cytotoxic (also known as killer) T-cells, can find and kill an infection. Memory B-cells also help bodies produce antibodies to prepare for possible future infections.

Based on the current evidence so far on COVID-19, we know that if the initial infection is more severe, immunity will typically last longer: re-infection might still be possible, but it will be far less likely to result in another severe infection.

Our immune systems have two parts: the innate immune system and the adaptive immune system. The innate immune is what we have at birth and it's the first responder first when a virus like SARS-CoV-2, the novel coronavirus that causes COVID-19, infects us. This part of the immune system tries to prevent the virus from spreading and reproducing in our bodies, and from moving around in our bodies.

The innate immune system is made up of several types of defenses, including skin and body openings like the mouth and nose, different white blood cells to defend our bodies from pathogens, and substances in bodily fluids and blood that try and stop the virus from reproducing. If the innate immune system is unable to kill the virus, the adaptive immune system activates to defend the body.

The adaptive immune response, also called the acquired immune system, consists of white blood cells: B cells and T cells. B cells create antibodies, which help us fight off viruses. T cells try to kill virus-infected cells like an army fighting off an invader. Some T cells also help B cells make antibodies, while others are busy working to stop the virus from reproducing in your body and spreading to different parts of your body.

Another key function of the adaptive immune system is immune memory. After the initial or primary response to a virus, the body mounts a secondary response, which involves a subset of B-cells and T-cells (known as memory cells) that help to create a longer term memory of the virus. This helps launch a faster defense in the event of a future infection.

Multiple early-stage studies have reported strong memory T-cell responses to the virus that causes COVID-19, which suggests that infection may prevent against recurrent, severe episodes of COVID-19. Researchers are still studying how long this memory-based immunity lasts and how strong it is in defending against COVID-19 infection in the future.