Difficulty of Symptom-Based Screening for COVID-19 and Onset of Flu Season Highlight Importance of Diagnostic Tests

By the bioMérieux Connection Editors

The arrival of the 2020-2021 flu season could substantially add to the difficulties caused by the COVID-19 pandemic. While community mitigation measures such as mask usage and social distancing helped reduce the rates of flu infection in the southern hemisphere and helped to reduce the impact of the 2019-2020 flu season overall, there is still reason for concern in the northern hemisphere. Although the CDC considers the flu shot to be the best way to prevent flu infection and recommends that the vast majority of people should receive one, surveillance data also show that flu is circulating at higher rates in locations where fewer community mitigation measures are in place.

The confluence of COVID-19 with flu season in communities where both viruses are circulating presents significant challenges with diagnosis especially. While there has been substantial interest in the ability to diagnose COVID-19 based on symptoms, not all symptoms have been described or are well understood. The signs of COVID-19 overlap significantly with other respiratory illnesses, and researchers have so far been unable to define a symptom profile that consistently differentiates COVID-19. The only symptom that researchers have identified as a potential differentiator for COVID-19 is a loss of smell and taste—but it is still not clear how frequently patients experience this. Even machine learning models have difficulty accurately predicting whether a patient has COVID-19 based on symptoms alone.

This lack of specificity is not unique to COVID-19. Severe respiratory infections generally have similar or overlapping symptoms, so diagnostic testing is often needed to determine the causative pathogen. However, the lack of differentiation between COVID-19 and flu is particularly concerning with the start of this flu season. The New York Times specifically notes that in the case of flu and COVID-19, “Most symptoms of the two diseases are so similar that, short of a test—or two or three tests—it won’t be possible to know for sure.” Additionally, there have been reports of coinfection, further complicating the picture.

Considerations in Testing for Flu and COVID-19

There are many different types of tests available for both flu and COVID-19, each with their advantages and disadvantages. In a typical flu season, many clinicians rely on Rapid Influenza Diagnostic Tests (RIDTs), which return results within a few minutes. The advantages of RIDTs are primarily speed and ease of use, and their disadvantages include sub-optimal sensitivity and limitations on viral type, subtype, and strain. In cases where the likelihood of coinfection is low and the proper interpretation algorithms are applied for high or low rates of infection, RIDTs serve their function very well. This year, the infection landscape is different, and testing strategies need to reflect that.

Two major concerns are the presence of COVID-19 and the variability of community mitigation measures, which affect the levels of flu virus circulation. Under these circumstances, local infection surveillance data will be very important and can help clinicians with interpretation of diagnostic test results and assessment of an individual’s risk for infection. Selecting which diagnostic tests to use will also play a role. Some tests look for a single pathogen, while others can check for multiple pathogens at the same time. Multiplex tests that check for COVID-19, flu, and other respiratory pathogens simultaneously could be especially helpful this flu season.

In addition to clinical needs, choosing tests depends on the level of complexity a given laboratory can handle. Laboratories with more resources and robust technology will be able to utilize tests that smaller laboratories cannot. Having a range of tests on the market that can meet the needs of large and small laboratories is always important, but especially so in circumstances like these where laboratory capacity is under significant strain. Regardless of the tests they choose, laboratories need to be ready to provide clinicians with the information they need to diagnose patients with respiratory symptoms quickly and accurately.

Distinguishing between COVID-19, flu, and coinfection is crucial because of implications for treatment. MedPage Today noted that, “dexamethasone is recommended for severe COVID-19 infection in hospitalized patients, but corticosteroids actually prolong viral replication in influenza.” Knowing whether a patient has COVID-19, flu, or both, will help clinicians provide appropriate treatment, improving patient outcomes.


Opinions expressed in this article are not necessarily those of bioMérieux, Inc.

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