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Does COVID-19 test positivity still matter? Vaccines are upending trusted virus metrics

Ben Conarck, Miami Herald on

Published in News & Features

MIAMI — About four out of every 10 Floridians have received at least one dose of a COVID-19 vaccine — a shift in the pandemic landscape that has upended the meaningfulness of various statistics that health experts, government officials and the public have relied on for the last year.

Vaccines haven’t just cut the number of people at risk of developing severe disease. They’ve also skewed the pool of who’s still getting tested for the virus, impacting still closely watched numbers like daily case counts and the percentage of tests coming back positive.

Some things haven’t changed. Hospitalizations from the virus have always been the most reliable way to gauge spread in a community. Now, the number of people hospitalized has become the single most important measure in understanding both the severity of an outbreak and how effectively the vaccines are working.

“The relationship between each of these metrics and what they mean for the future is rapidly changing,” said Stephen Kissler, an immunology and infectious disease expert with Harvard T.H. Chan School of Public Health.

One of the foremost statistics used to measure spread of the virus, test positivity, has been undercut by the shifts in who is getting tested and vaccines, according to experts interviewed by the Miami Herald.

“I don’t think positivity is very useful anymore,” said Eric Toner, a senior scholar at the Johns Hopkins Center for Global Health Security.


Early in the pandemic, Toner said, positivity was an especially crucial statistic, because most cases were being missed.

That changed during Florida’s fall coronavirus lull, when access to testing expanded following a summer surge and record positivity rates. Heading into the winter, Florida’s case counts were more accurate and timely, and the positivity rates became useful again, but for a different reason.

By late summer, positivity became essential for detecting increases in spread, such as the rise in cases following holiday gatherings, when testing levels had reached their heights, and most recently again in early to mid-March.

Kissler, the Harvard infectious disease expert, said test positivity will still be a good measure to detect subtle increases in spread signaling a future rise in cases, but it no longer serves as a valid comparison point to earlier in the pandemic.


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