The US's number of new COVID-19 cases grew by 27 percent last week compared to the week before.

President Donald Trump has suggested that the higher numbers are simply the product of increased testing, since more widely available diagnostic testing leads more cases to be caught and confirmed.

The US has significantly ramped up testing – it went from testing about 110,000 people per day on March 29 to about 640,000 on July 4. But that is not the primary reason for the recent spike in cases.

"The surge numbers are real," Irwin Redlener, director of the Columbia University National Centre for Disaster Preparedness, told NBC News.

Although testing does capture more cases, he said, "to deny the fact that we're having an ongoing pandemic with continued spread is contrary to all evidence that we have and everything that we know about the behaviour of the virus."

One metric can easily show whether increased testing is to blame for higher case counts: the positivity rate, the portion of diagnostic tests that come back positive over a given period of time. If testing is increasing and the number of new daily cases in a community is staying flat, the positivity rate should go down.

But that's not the case in 27 states, where positivity rates are going up, according to data from Johns Hopkins University.

In Idaho, for example, the average positivity rate from July 1 to 7 was 12 percent, where as it was 7 percent from June 14 to 21, and just 3 percent from June 1 to 7.

In South Carolina, the seven-day positivity rate is around 17 percent. A month ago, on June 7, it was 6 percent.

This graph shows how six states' positivity rates have changed over time. The charts show seven-day rolling averages, meaning the rate shown on each day is an average of the previous week's positivity percentage.

graph of tested positive cases covid july bi(Yuqing Liu/Insider)

Rising positivity rates indicate that the virus' spread is accelerating within a community, and that an outbreak could be spiraling out of control. Generally, the spikes in the charts in March reflect the period of time when testing was limited and tests were mostly reserved for symptomatic patients with severe cases.

Positivity rates tell the truth about an outbreak

The World Health Organisation recommends that governments institute a stay-at-home order if more than 5 percent of tests are coming back positive in a given region.

That's because a positivity rate over 5 percent indicates that the virus is spreading significantly through the community.

"This target is based on the experience of countries that have driven their numbers of cases down and largely stopped viral spread. Many places, such as South Korea, Australia and New Zealand have been keeping their positivity much lower – at 2 percent or less," Jennifer Nuzzo, an epidemiologist with the Johns Hopkins school of public health, wrote in a Washington Post op-ed.

Currently, however, 28 US states are seeing positivity rates above that threshold.

Arizona, whose total case count passed 50,000 on June 21 then doubled to more than 100,000 in the following two weeks, had a positivity rate of 25 percent over the last week. That's the highest in the US.

Florida comes in second, at 19 percent, followed by South Carolina (17 percent), Mississippi (15 percent), and Texas (14 percent).

Five more states have positivity rates over 10 percent: Nevada (13 percent), Georgia (13 percent), Alabama (12 percent), Idaho (12 percent), Kansas (10 percent),

Hongjie Liu, chair of the department of epidemiology and biostatistics at the University of Maryland, told the Virginia Mercury in May that positivity rates serve as an indicator of how fast the virus is spreading.

"10 percent indicates an ongoing epidemic," he said. "Meaning the epidemic is spreading relatively fast. And 10 percent is not a small number. It's a huge number in any epidemic."

That high a positivity rate, he added, "implies that the risk for other people to acquire the virus is high."

States use positivity rates to make public-health decisions

When positivity rates rise over time in a particular region, that's a sign an area is becoming a hotspot. By that logic, some states are using positivity rates as a metric to determine whether it's safe to reopen businesses, transit, and public spaces.

In early May, Texas Gov. Greg Abbott said any positivity rate over 10 percent was cause for alarm; when the state's overall rate surpassed that at the end of June, Abbott paused Texas' reopening plan. California considers a positivity rate of 8 percent to be a safe level for counties to reopen.

Positivity rates go down, of course, once an epidemic is under control and fewer people start testing positive, or when testing expands at a rate faster than the virus can spread.

However, the US faces major hurdles in ramping up testing to match this new surge in cases. Many states face a shortage of necessary supplies and delays in processing tests due to backlogs at laboratories.

Meanwhile, the country is on the cusp of a grim new milestone: 3 million infections.

This article was originally published by Business Insider.

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