Covid-19 case rates still matter

By Deidre McPhillips, CNN

Covid-19 cases in the United States have fallen more than 95% from their peak in January, and on Friday, Hawaii will become the last state to roll back its indoor mask mandate. Testing has slowed, and states are scaling back on reporting surveillance data, too.

But experts say that tracking new Covid-19 cases is still important — even if it is an imperfect measure.

“Dating back to the beginning of the pandemic, we’ve always known that case numbers were being undercounted. It was never quite clear how much,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

With at-home tests more widely available and vaccination helping prevent severe disease, people might be skipping laboratory-confirmed PCR tests. Before, limited availability of testing probably created similar gaps in reporting. The US Centers for Disease Control and Prevention estimates that even before Omicron, only 1 in 4 cases was officially reported.

“With an infection that can rapidly spread, where we are today in the data isn’t necessarily going to be reflective of how bad it’s going to be tomorrow or the day after,” said Michael LeVasseur, professor of epidemiology and biostatistics at Drexel University’s School of Public Health. “There could be many, many more people who are infectious that you can encounter that you’re not seeing in the data.”

But even as they fall, daily case counts provide a useful benchmark in a more comprehensive assessment of the situation that calls for a look across multiple trends.

“From a public health perspective, it’s always important to look at more than one data source. And when those data sources together are pointing in the same direction, you can have more confidence,” said Janet Hamilton, executive director of the Council of State and Territorial Epidemiologists.

Hospitalizations and deaths have consistently been the most reliable Covid-19 metrics, experts say.

“Those two are bookends that don’t really change much relative to the availability of testing,” Osterholm said.

Early indicators of transmission have more room for improvement. Case rates are best understood in tandem with test positivity rates. Visits to emergency room departments and virus levels in wastewater can help, too.

Despite the dramatic improvement over the past two months, coronavirus transmission is still high.

“Even though things are going down, boy, is there a lot of Covid out there,” Hamilton said.

Once driven solely by transmission levels, the CDC adjusted its guidelines for indoor masking last month to rely on hospitalization metrics more heavily than cases.

Though the CDC’s newer “Community Levels” map is mostly green, the original “Community Transmission” map still has lots of red. As of Thursday, less than 2% of US counties have a “high community level,” but more than a third of counties still had “high” or “substantial” transmission over the past week.

Experts admit that balancing all of these metrics is a complicated risk assessment for individuals to make day to day and that public messaging to help has missed the mark.

“I don’t think we have hit our sweet spot for how we’re helping communicate that to people,” Hamilton said.

“It’s hard, because I think it’s not only important to look at a metric which says, ‘Yes, there’s virus out there.’ It’s also important to apply that to your own individual risk level.”

And this individual risk assessment might get harder in the days and weeks to come if signals become mixed. In the UK, for example, the BA.2 subvariant of Omicron led to a rise in cases that has been much sharper than the rise in hospitalizations or deaths.

But an individual approach shouldn’t necessarily change, whether there are 500 new cases per day or 50,000 cases per day, LeVasseur said.

“I can take a look at trends and do a back of the envelope guess at how I’m feeling about risk. But it doesn’t stop me from wearing a mask. It doesn’t stop me from having conversations with the people that I’m going to be with about our approach,” he said.

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