New study reveals that asymptomatic infections are important drivers of influenza transmission

A new study, published in Lancet Global Health, was conducted by the National Institute for Communicable Diseases (NICD), Perinatal HIV Research Unit (PHRU), WITS Agincourt HDSS in partnership with the US Centres for Disease Control and Prevention (CDC), who also funded the study. The study evaluated the community burden and transmission of influenza (flu) from 2017 to 2018 in a rural and an urban South Africa community.

Approximately 100 households, randomly selected from a rural and an urban setting in South Africa, were enrolled each year and observed for a period of 10 months. The data was collected through systematic twice-weekly nasopharyngeal sampling of all household members and samples were tested for the presence of influenza by polymerase chain reaction (PCR). A total of 81,430 samples were collected from 1,116 participants in 225 households, of which, 917 (1%) tested positive for influenza and 79% of households (178/225) had ≥1 influenza-positive individual.

The study revealed that the burden of laboratory-confirmed flu was high in a rural and an urban African setting, with over three-quarters of households and more than one in three individuals experiencing at least one flu infection each year. It is important to note that the flu incidence risk was similar between the rural and urban areas who participated in the study. Interestingly, repeated flu infections within the same annual flu epidemic, particularly in children, were a common occurrence in more than 15% of those infected. With young children also experiencing the highest burden of flu infection and symptomatic illness. This group was more likely to spread the flu to others in their household, in comparison to other age groups.

The study furthermore revealed that just over half of the flu infections were symptomatic, indicating that a high number of asymptomatic flu infections were present. Asymptomatic individuals were also able to spread flu, transmitting the flu to approximately 6% of household contacts. For this reason, authors of the study believe asymptomatic infections to be an important driver of flu transmission.

Medically attended influenza-associated influenza like illness (ILI), defined as a fever and cough as captured by the World Health Organization-recommended flu surveillance programs, suggest that the flu disease burden in the community may be substantially higher than that observed through facility-based surveillance. Understanding the community burden and transmission of seasonal influenza is critical to guide vaccination programmes and non-pharmaceutical interventions, not to mention informing pandemic preparedness.

In conclusion, the study provides important data on the community burden of flu and transmission thereof in an African setting, a topic that hasn’t been adequately explored. It also contributes important findings relating to symptomatic and asymptomatic flu transmissions, and has implications for the use of non-pharmaceutical interventions and vaccination strategies that target children.

A similar study to examine the burden and transmission of SARS-CoV-2 in the same communities including the role of asymptomatic infections in the spread of SARS-CoV-2 was initiated in July 2020 and results of this study are expected in the coming months.


For more information, contact Professor Cheryl Cohen:

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