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Clinical Laboratories May See Unexpectedly Low Number of Influenza Cases This Flu Season

cdc influenza testing positive update

Influenza cases are on the decline, but will this actually be beneficial for clinical laboratories?

Influenza season has arrived in the Northern Hemisphere, but early indicators are that this flu season may be very mild compared to typical expectations. Flu season in the United States has started slowly, according to federal data. Other countries appear to be experiencing similar impact compared to previous years.

The timing of flu season differs in the Southern Hemisphere, where the concentration occurs six months earlier than in the United States. In this area, flu rates were also lower than normal for flu season. Said Kanta Subbarao, MD, Director of the World Health Organization’s (WHO) Collaborating Centre for Reference and Research on Influenza in Melbourne, Australia, as part of an August 2020 NPR interview, “Based on what we’ve seen in the Southern Hemisphere—and I would say this is true of all through the Southern Hemisphere—South America, Africa, Australia, New Zealand, all across this region, there’s been very little influenza activity.”

Examples of Flu in 2019 Compared to 2020

In the United Kingdom, Public Health England recently released data that showed “there were only 1.2 GP consultations for suspected flu per 100,000 people last week. This time last year—a relatively mild year for flu—the figure was 10.6 per 100,000, so 90% higher.”

Officials in Canada say they have seen a similar reduction in normal influenza levels in 2020 compared to 2019. On its website, the federal Canadian Health Authority recently stated, “All indicators of influenza activity remain exceptionally low for this time of year, despite continued monitoring for influenza across Canada. To date, there is no evidence of community circulation of influenza.”

Recent Centers for Disease Control and Prevention (CDC) data align, showing that influenza levels in the United States also have remained low. “The percentage of visits to a health care provider for influenza-like illness (ILI) remained at 1.6% for a third consecutive week,” says the CDC’s weekly U.S. influenza surveillance report website.

Moreover, the CDC notes in a recent report that influenza does seem to be on a sustained decline. “The global decline in influenza virus circulation appears to be real and concurrent with the COVID-19 pandemic and its associated community mitigation measures,” says the CDC report. “Influenza virus circulation continues to be monitored to determine if the low activity levels persist after community mitigation measures are eased. If extensive community mitigation measures continue throughout the fall, influenza activity in the United States might remain low and the season might be blunted or delayed.”

Decline in Seasonal Flu Has Positive Impact on Clinical Laboratories?

A recent comparison of the total number tested for flu by US clinical laboratories shows a spike during the fall season of 2020, during weeks 46 and 47 to date. Charts 1 and 2, below, show testing by clinical laboratories compared to public health laboratories, according to CDC data.

snapshot influenza testing in clinical laboratories CDC data December 2020

snapshot of influenza testing in public health laboratories December 2020
Charts 1 and 2, above, show testing by clinical laboratories compared to public health laboratories, according to Centers for Disease Control and Prevention (CDC) data. These charts show partial-year information. The full view of week-by-week comparison of influenza testing activity in clinical laboratories and public health laboratories can be found here.

Globally speaking, the decrease in influenza cases might seem to be welcome news to clinical laboratories that are struggling with supply chain issues at the same time as increased demand for COVID-19 testing. A decrease in influenza testing could help clinical laboratories to avoid strain on testing supplies such as swabs and reagents that may also be needed for COVID-19 testing.

However, a decreased number of influenza cases might not actually lead to a decrease in influenza testing. It remains to be seen how this change from normal epidemiological patterns will actually impact physician orders, but preliminary data indicates that the number of influenza tests ordered may be higher than last season in spite of the decreased number of flu cases. Surveillance data from 2019 show the total number of specimens tested by clinical laboratories (for flu) during week 46 was 24,405, compared to 34,708 this year. At week 47 in 2019, the total number of specimens tested for flu was 21,367, compared to 37,555 this year.

Influenza is an obvious differential diagnosis for COVID-19, especially during the time of year that is traditionally influenza season. Doctors may continue to order a high number of influenza tests for COVID-19 patients to rule out influenza as a coinfection.

While a decreased number of flu cases may seem promising, clinical laboratory managers should consider that this benefit may be offset by the increased need for influenza testing for patients with COVID-19 symptoms.

Caleb Williams

—By Caleb Williams, Editor, COVID-19 STAT

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