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Can a person test negative and later test positive for COVID-19?
Viral tests are used to gauge whether you have a current infection. A negative result means that the virus that causes COVID-19 was not found in the person's sample. This probably means that the person is not infected with the coronavirus. However, false negatives can occur for a number of reasons, including people being tested too soon after exposure to the virus (which may not let enough of the virus build up to a level that is detectable), differences in how well the coronavirus is able to make copies of itself in one person compared to in another person, and cases where sample collection (e.g. getting swabbed) does not, for whatever reason, capture enough of the virus.
One study from researchers at Johns Hopkins suggested that COVID-19 PCR tests conducted 3 to 5 days after a person is exposed that return a negative result shouldn't be relied on alone to gauge infection status, and that in addition, the clinical and epidemiologic situation should be carefully evaluated by health professionals. The study also found that the false-negative rate is at a low point about 8 days after exposure.
So if people think they've been exposed to COVID-19, they should quarantine, connect with a healthcare professional to map out a plan for when to get tested or take other steps to address their health, and monitor for symptoms.
A negative test result for COVID-19 for a sample collected while a person has symptoms likely means that the COVID-19 virus is not causing their current illness.
Updated 3 December 2020