The growing concern of the influenza H5N1 virus

3 December 2024

The H5N1 influenza virus, also known as bird flu, has been a topic of concern for several years due to its potential to infect humans. While it primarily affects birds, there have been over 900 documented cases of human infections since 2003. The primary concern surrounding the H5N1 influenza virus is its potential to spread from person to person.

The symptoms of H5N1 infection in humans are similar to those of other influenza viruses. They include fever, sore throat and cough. However, many cases have reported more severe symptoms such as pneumonia, acute respiratory infection, and even death. The severity of H5N1 infection in humans is indeed a significant concern. According to the World Health Organization, more than half of reported human cases have been fatal, resulting in over 450 deaths since 2003, making it one of the deadliest flu viruses known to date. Yet, none of the 55 infected cases in the recent outbreak in the United States was fatal, suggesting an incomplete picture of the risk associated to this virus.

The US Centers for Disease Control and Prevention has stated that the risk of human-to-human transmission is low, but not impossible. If the virus were to become easily transmissible between humans, it could lead to a new pandemic similar to what the world experienced in past years, like in 2009 with the H1N1 pandemic, in 1977 or with the deadly 1918 Spanish flu. Transmission from birds to humans is a rare event. Instances of transmission to humans from other mammals (pigs, or cattle) which, like humans, may become infected as a result of close contact with infected birds, have also been reported: in the recent outbreak in the United States, almost all cases where traced back to close contact with cattle. There has been no documented instance of direct human to human transmission and, at the moment, the virus seems unable to sustain its transmission among the human population. Yet, because of the ability of influenza viruses to rapidly mutate, and owing to the high prevalence of this virus in wild birds, the risk cannot be ignored.

At Zetesim, we are developing models for influenza, including a potential H5N1 threat. In the event that H5N1 is recognized as a major threat, we believe to be well positioned to contribute useful information to individuals, with our forecasting capabilities, to help take informed decisions. One of the unknowns surrounding H5N1 is the number of asymptomatic cases. Estimates based on the number of cases showing no symptoms that have been detected in screening campaigns suggest that up to 50% of infected individuals may not show any symptoms. However, this estimate is based on limited data mostly obtained by testing close contacts of infected individuals: more research is needed to confirm this figure. The fraction of asymptomatic cases is an important parameter in models used in forecasting.

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