The answer to this common question is a bit more complex due to all of the factors that go into becoming “immune” from future infections.
In general, a person with demonstrated IgG antibodies to the pathogen in question has a level of protection or immunity that can help protect them against exposure and future infections to that pathogen. Therefore, a person who tests positive for COVID-19 IgG antibodies can be reasonably reassured that they have some level of protection against COVID-19 in its current form.
However, the level of your immunity, and how long it is likely to last, are also dependent upon the amount of antibodies your body produces in response to the pathogen during a future exposure. Also, your immune system’s ability to respond to the pathogen in the future may diminish over time. This is why, for instance, for those that have been vaccinated for certain diseases (such as tetanus) a periodic ‘booster shot’ may be required to stimulate the immune system to produce a new level of antibody protection that might have otherwise diminished over time.
Importantly, some pathogens have the ability to change or mutate certain components such as their outer proteins or antigens. Therefore, the virus that causes COVID-19 could change over time, thus making even a robust immune response to the original virus less effective against the new strain.
In response to an infection, such as COVID-19, the body develops an overall immune response to fight the infection. One component of the immune system's response is development of antibodies that attach to the virus and help eliminate it. The body's initial immune reaction produces general antibodies that attack many infections, called "IgM" antibodies. IgM antibodies indicate an active or recent infection. Because it takes time for the body to make IgM antibodies in response to SARS-CoV-2, the absence of IgM antibodies does not mean that the person is not infected. A test for IgM antibodies might also give a false negative result, i.e., the test is negative in a person who in fact has IgM antibodies or is starting to make them but has not made enough to be detectable on the test yet. This can occur in a patient with SARS-CoV-2, particularly during the early stage of the infection, while the person is actively shedding the virus. Since IgM antibodies may not
develop early or at all in infected patients, this type of antibody test is not used to rule out SARS-CoV-2 in an individual.
Over time, the body develops a second type of antibody in response to the infection. This antibody, which is more specific to the virus, is called an "IgG" antibody. Most antibody tests detect IgG antibodies. On average, IgG antibodies take a few weeks after an infection to develop, but the time to development may vary substantially, and there is still a lot we do not know about SARS-COV-2. Since IgG antibodies generally do not develop until several weeks after infection, this type of antibody test, even though it is more specific to SARS-CoV-2, is not used to rule-out SARS-CoV-2 infection in an individual.