How Flu Virus Shedding Can Predict Contagion

Viral shedding during influenza infections has 2 traits of interest to researchers: the quantity of virus shed and the pattern in which infected individuals shed virus over time.

When infected individuals shed large quantities of virus, researchers predict widespread flu, as shed quantity is a proxy for infectiousness.

Shed pattern indicates how long individuals are able to “share with others,” and linking symptomatology to shed pattern helps predict the average duration of illness.

To date, most viral shedding studies have been done in the laboratory, but understanding the relationship between clinical symptomatology and viral shedding in naturally acquired influenza infections could help researchers translate their findings more accurately to the general population.

An article published in the journal Clinical Infectious Diseases does just that.

Researchers screened healthy individuals in 824 households between 2008 and 2014 for naturally acquired influenza virus infection. They identified 224 secondary cases of flu and followed the dynamic relationship between patterns of clinical symptomatology and viral shedding in 127 people who developed acute respiratory infection.

Among the study subjects, the researchers documented a subset of 17 cases of pandemic influenza A(H1N1), 43 of seasonal influenza A(H1N1), 43 of influenza A(H3N2), and 24 of influenza B.

Cough and runny nose heralded the flu’s arrival for 70% to 80% of patients, and these symptoms continued over the infection’s course for 70% to 80% of patients regardless of the type of infection.

Systematic symptoms such as aches and discomfort tended to resolve faster than cough and other respiratory symptoms.

Viral shedding in influenza A virus infections peaked on the first 1 to 2 days of symptomatic illness. By day 6 or 7, viral shedding had gradually decreased to undetectable levels.

Patients who contracted influenza B exhibited peak viral shedding up to 2 days before symptom onset. Shedding then declined and peaked again at day 3 to 5, and then persisted over 6 to 7 days.

This means that symptoms indicate clinical infectiousness for influenza A virus, but for influenza B virus infections, patients are probably sharing the virus before symptoms become evident.