Trump’s Covid-plagued White House proves testing alone can’t keep America safe

President Donald Trump’s White House continues to rack up positive tests, from Hope Hicks to Stephen Miller. On Tuesday, press secretary Kayleigh McEnany (and two of her aides) tested positive for SARS-CoV2, the coronavirus that causes Covid-19. Despite knowing that she was in close contact with both the president and top adviser Hope Hicks prior to their diagnoses, McEnany not only refused to quarantine, but even continued to give briefings with reporters without a mask.

Her stated reason for this behavior, up until Monday afternoon? She hadn’t tested positive yet.

This behavior highlights a fundamental and dangerous misunderstanding of the point of Covid-19 tests — and their limitations. If we don’t know how to interpret and respond to tests, we risk the kind of disaster now unfolding at 1600 Pennsylvania Ave.

To be clear, testing is not a prevention strategy. Just like a pregnancy test cannot take the place of birth control, Covid-19 tests should not be seen as substitutes for robust strategies to reduce community transmission. It is part of the public health approach, but not for the reasons people think.

The first and most familiar reason people get tested is to obtain a definitive diagnosis. This type of testing is most often done for individuals with symptoms — people who have a cough, fever, loss of taste or smell, or fatigue. Asymptomatic contacts of a known Covid-19 positive patient may also be tested to rule out infection. This type of testing is usually done with a “PCR” test, which amplifies the virus and which is currently considered the gold standard for testing, although other more rapid forms of molecular tests can also be used. Most tests on the market were approved for this type of “diagnostic” testing, and it has the most robust metrics.

The second reason to test is to monitor the disease on a population level. Individual-level accuracy is less important here. The goal is to detect positive cases before they spread. In this type of testing, we want to test frequently and widely; this is the strategy used by universities, sports teams and workplaces. Many groups are using rapid antigen tests, which are quicker but less accurate, and which have not (for the most part) been proven particularly effective in identifying asymptomatic patients; others are using novel strategies like “wastewater testing” (e.g., testing sewage).

Testing is also used to see if someone has recovered from an infection. For this purpose, an antibody test is used to see if you have mounted an effective immune response; or a repeat PCR test is used to see if you have eliminated the virus from your body. These tests are not completely accurate, though, and should not be used as the sole marker of whether it’s safe to be around other people.

The last main reason we test is to try

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