[[File:Influenzastructure.png|thumb|360px|Structure of the influenza virus.<ref name="Arti1">Host Protective Immune Responses against Influenza A Virus Infection, MDPI, 2020, Hi Eun Jung, Heung Kyu Lee, https://www.mdpi.com/1999-4915/12/5/504/htm</ref>]]

The theme of SensUs 2021 is acute respiratory viruses. The current Covid-19 pandemic has made it apparent that large virus outbreaks can cause immense harm to human health and can disrupt society as a whole. One of the most common respiratory viruses is influenza A. Therefore, the influenza virus serves as an interesting biomarker for this year’s Competition.

These two proteins are a target of interest for antiviral drugs.<ref name="Arti15">Recent Strategies in the Search for New Anti-Influenza Therapies, Current Drug Targets , 2003, J.C. Wilson, M. von Itzstein, https://www.eurekaselect.com/63785/article</ref> Furthermore, they are also the antigen proteins to which a host antibodies can bind and trigger an immune response. Influenza type A viruses are categorized into different subtypes, or strains, based on which type of these two proteins is present on the surface of the virion. Currently, there are 16 subtypes of HA and 9 subtypes of NA known to exist. The most prevalent form of the different subtypes is H1N1. Single hemagglutinin-neuraminidase proteins, in which both HA and NA are found in a single protein, also exist. However, these will not be used in SensUs 2021.

[[File:Flu-infect.jpg |right|thumb|200px|An influenza virus infecting a cell.<ref name="Arti31">Sugars on Cell Surface Are Key to Flu Infections, National Institute of Health, 2008, https://www.nih.gov/news-events/nih-research-matters/sugars-cell-surface-are-key-flu-infections</ref>]]

After infection is complete, the H1N1 virus triggers cell apoptosis, leading to the death of the cell and spread of the virions.

The influenza virus spreads when people with the flu cough, sneeze or talk. Infected people transfer tiny droplets to people close to them and infect them by the handover of droplets. During an outbreak, it is important to be able to control and prevent the virus from spreading further, for example, by implementing control measures. Those control measures can be determined based on the reproduction number. The reproduction number is defined as: “the expected number of secondary cases produced by a single infection in a completely susceptible population”.<ref name="Arti16">Notes on R0, 2007, James Holland Jones, https://web.stanford.edu/~jhj1/teachingdocs/Jones-on-R0.pdfe</ref> A clear overview of the number of infected people is needed to calculate the reproduction number.

When working with the influenza virus, influenza infection in humans can occur following a laboratory accident. For safety reasons, inactivated virus particles will be used in SensUs 2021, as a substitute for infectious influenza virus particles.

The use of safety equipment combined with good practices is fundamental to laboratory safety and in helping to reduce the risks involved in dealing with biosafety hazards. Therefore, it is important that you consult with your local biosafety officer and comply to the safety rules of your own organization.