evidence: Yet COVID-19 tests aren’t easy to come by everywhere. Consumers have complained that their pharmacies are sold out and that testing centers in some places have long lines. And even if you can get your hands on a test, many folks — like my friend — have questions. In particular, she wanted to know if her daughter’s recent vaccine shot would interfere with the results of her home test. [...] There are two major categories of diagnostic tests you might encounter: antigen tests and nucleic acid amplification tests, better known as PCR tests. Both can determine whether you have a current infection with SARS-CoV-2, the virus that causes COVID-19. You can find a list of tests available in stores or at testing sites in the United States here. (Antibody tests aren’t diagnostic tests: They can detect evidence of past infections, but not current cases.) [...] At-home kits aren’t as sensitive as the PCR tests, but "the rapid antigen tests are really great, particularly if you’re symptomatic, to determine whether you have COVID at that moment," says Gigi Gronvall, an immunologist and senior scholar at the Johns Hopkins Center for Health Security. "They rival PCR for accuracy for that time" when people are symptomatic and most likely to be contagious to others.