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Viral shedding of influenza B begins before symptom onset, after improvement
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Viral shedding of influenza B occurred before the onset of acute respiratory symptoms and after clinical improvement, suggesting illness profiles may not amply represent infectiousness, according to a recent study.
“As a consequence, symptom-based preventive measures may be inadequate, while more effective control of epidemics may be achieved through general preventive measures including hygiene campaigns, improvements in general immunity by healthier lifestyle and the use of influenza vaccination, especially for individuals of high-risk of severe disease,” Dennis K.M. Ip, MPhil, MSc, of the School of Public Health at the University of Hong Kong, and colleagues wrote.
Ip and colleagues examined data on 127 initially healthy patients from Hong Kong who developed influenza from a household contact between 2008 and 2014. The researchers collected nose and throat swab samples during three household visits and assessed self-reported illness profiles to observe the relationship between symptomatology and viral shedding. The samples underwent reverse-transcription PCR testing and quantitative viral dilutions to determine the median tissue culture infectious dose and viral load replication.
The analysis included 17 cases of pandemic influenza A(H1N1), 43 cases of seasonal influenza A(H1N1), 43 cases of influenza A(H3N2) and 24 cases of influenza B. Although the symptom patterns in influenza A and influenza B infections appeared to be similar, the relationship between symptoms and viral shedding varied, according to researchers.
For influenza A infections, mean viral shedding was detected before the onset of symptoms and peaked within 2 days of acute respiratory illness. The amount of shedding gradually decreased to an undetectable level by day 6 or 7 of clinical illness. In contrast, viral shedding of influenza B infections peaked up to 2 days before symptom onset and lasted for 6 to 7 days, indicating a bimodal pattern.
“While clinical illness profiles may serve as a proxy for clinical infectiousness in influenza A virus infections, patients may potentially be infectious even before symptom onset or after clinical improvement in influenza B virus infections,” Ip and colleagues wrote.
Further analysis showed a strong correlation between body temperature and viral shedding for influenza A on the day before and the day immediately after symptom onset; however, the relationship was not observed in influenza B infections.
In a related editorial, Jeffrey C. Kwong, MD, MSc, from the University of Toronto and Public Health Ontario, said more research is needed to determine whether transmission occurs during the pre-symptomatic phase or if patients with fewer than two symptoms can transmit disease. If the viral shedding pattern for influenza B is confirmed, he wrote, the findings would highlight the value of rapid, self-administrating diagnostic testing for influenza.
“One can envision a future when rapid diagnostic testing for multiple respiratory viruses could become widely available for the public use when they experience respiratory infections,” Kwong wrote. “If that time arrives, we will be able to more fully benefit from the findings of this study.” – by Stephanie Viguers
Disclosures: Ip reports research funding from Hoffmann-La Roche. Kwong reports being supported by awards from the Canadian Institutes of Health Research and the University of Toronto Department of Family and Community Medicine and receives salaries from the Institute for Clinical Evaluative Sciences and Public Health Ontario. Please see the full study for a list of all other authors’ relevant financial disclosures.
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