When the Omicron variant of the coronaviruses arrived in the United States last fall, it pushed new case numbers to previously unseen peaks.
The record wave of recorded infections was a significant undercount of reality.
In New York City, there were more than 538,000 new cases in the first three months of the year, representing 6 percent of the city's population. According to a recent survey of New York adults, there could have been more than 1.3 million additional cases that were never detected or never reported, and 27 percent of the city's adults may have been exposed.
The official tally of coronaviruses infections in the United States has always been underestimated. Scientists say that case counts are becoming unreliable as states shutter mass testing sites and institutions cut back on testing.
It seems like the blind spots are getting worse with time, according to Denis Nash, an epidemiologist at the CUNY Graduate School of Public Health and Health Policy who led the New York City analysis.
That could leave officials in the dark about the spread of the highly contagious new subvariant of Omicron known as BA.2, he said.
As BA.2 spreads, the official case count can pick up major trends, and it has begun to tick up again. Experts said that undercounts are likely to be a bigger problem in the weeks ahead, and mass testing sites may never return.
The reality is that we don't really have eyes on it.
To track BA.2 and future variants, officials will need to look at an array of indicators, including hospitalization rates and wastewater data. Scientists said that keeping tabs on the virus will require more creativity and investment.
Some scientists said that people can gauge their risk by paying attention to people they know are catching the virus.
If you hear your friends and co-workers get sick, that means your risk is up and you probably need to be testing and masking.
Tracking the virus has been difficult since the beginning of the Pandemic. Many people didn't have the time or resources to get tested or find out if they had infections.
By the time Omicron hit, a new challenge was presenting itself: At- home tests had finally become more widely available, and many Americans relied on them to get through the winter holidays. Many of the results were never reported.
The groundwork hasn't been done to systematically capture those cases on a national level.
People can report their test results using digital tools. A recent study suggests that it may take a lot of work to get people to use them. Residents of six communities across the country were invited to use an app or online platform to order free tests, log their results and then send that data to their state health departments.
Almost 180,000 households used the digital assistant to order the tests, but only 8 percent of them recorded any results on the platform, and only a third of those reports were sent to health officials.
General Covid fatigue, as well as the protection that vaccination provides against severe symptoms, may prompt fewer people to seek testing, experts said. The federal government stopped reimbursing health care providers for the cost of testing uninsured patients because of a lack of funds, prompting some providers to stop offering those tests for free. Uninsured Americans are more likely to be reluctant to test, Dr. Jetelina said.
The poor neighborhoods will have more depressed case numbers than high-income neighborhoods.
If we see an increase in cases, that is an indicator that something is changing, and it is possible that something is changing because of a larger shock to the system.
It could take officials longer to detect new surges if more modest increases in transmission are not reflected in the case tally. The problem could be worsened by the fact that some jurisdictions have begun updating their case data less frequently.
Some of the challenges have been explored by Dr. Nash and his colleagues. To estimate how many New Yorkers may have been affected by the Omicron surge, they surveyed a diverse sample of 1,030 adults about their testing behaviors and results, as well as potential Covid-19 exposures and symptoms.
The cases that would have been caught by standard surveillance systems were those people who reported testing positive for the virus on tests administered by health care or testing providers. People who had Covid-19-like symptoms and had known exposures to the virus were included in the group of people who were counted as hidden cases.
The responses were used to calculate how many infections might have escaped detection.
The study has limitations. It excludes children as well as adults living in institutional settings. The same approach could be used by the health departments to fill in some of their blind spots.
He said that you could do these surveys on a daily or weekly basis.
Natalie Dean, a biostatistician at Emory University, said that replicating what Britain has done would be a good approach.
The method is expensive, and Britain has recently started scaling back its efforts.
The spread of Omicron, which causes milder disease than the earlier Delta variant, has prompted some officials to put more emphasis on hospitalization rates.
If our goal is to track serious illness from the virus, I think that's a good way to do it.
Hospitalization rates are lagging indicators and may not capture the true toll of the virus, which can cause serious disruptions and long Covid without sending people to the hospital.
There are different portraits of risk created by different metrics. In February, the Centers for Disease Control and Prevention began using local hospitalization rates and measures of hospital capacity, in addition to case counts, to calculate its new community levels, which are designed to help people decide whether to wear. According to this measure, more than 95 percent of U.S. counties have low community Covid-19 levels.
According to the C.D.C.'s community transmission map, only 29 percent of U.S. counties have low levels of viral transmission.
Hospitalization data may be different from place to place. Some localities are trying to distinguish between patients who were hospitalized specifically for Covid-19 and those who picked up the virus incidentally.
The chief medical officer at the New said that they needed to update their metrics because of the factors of the virus.
New Hampshire's Covid-19 online dashboard displayed all of the hospitals with active coronaviruses. The number of hospitalized Covid-19 patients taking remdesivir or dexamethasone is now displayed. Data on all confirmed infections in hospitalized patients is available through the New Hampshire Hospital Association.
Wastewater surveillance is one approach that doesn't rely on testing or health care access. Monitoring the levels of the coronaviruses in wastewater is an indicator of how widespread it is in a community.
Dr. Andersen is working with colleagues to track the virus in San Diego's wastewater.
Wastewater data from hundreds of sampling sites has been added to the C.D.C.'s Covid-19 dashboard, but coverage is not uniform, with some states reporting no current data at all. Scientists said that the data needs to be released in near real time if wastewater surveillance is going to fill the testing gaps.
Dr. Andersen said that water is a no-brainer. If we realize that we have to do that.
School closings, flight cancellation and geographic mobility are some of the data sources that officials can use.
Pulling those together in a thoughtful, coordinated way is one of the things we aren't doing a good enough job of.
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