People who have had COVID-19 develop antibodies to the infection in their blood, so tests can be designed to detect those antibodies and indicate if someone has ever had COVID-19. However, these tests are not always accurate. Tests can be done in laboratories, but simpler self-testing kits enable people to prick their finger to test their blood at home. Self-test kits show only a positive or negative result, with no further details. People may get a negative result even with a small number of antibodies in their blood. Just as home pregnancy test results are not always right, self-test kits for COVID-19 should never be used as definitive indicators of whether someone has had the virus. If someone gets a positive result at home, they should get further testing to confirm, and a negative result does not mean that they never had the virus. Scientists still do not know what an ideal immune response to COVID-19 looks like. Having antibodies may not prevent reinfection. And antibodies may gradually disappear from the blood over time—perhaps after a few months or years—potentially leaving that person vulnerable to infection again. Immunity to the seasonal flu tends to last for about a year, but we do not yet know if COVID-19 immunity will be the same, longer, or shorter.
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