Following the publication of a study in BMC Medicine investigating vaccination strategies to prevent the spread of flu, we asked co-author Marc Baguelin, a mathematical modeler of the influenza virus, to put together five facts about flu for you, just in time for the flu season!

### 1. Flu is particularly difficult to track

As a mathematical modeler with a mission to design strategies to control influenza, my first task is to understand flu transmission in the community. This is not an easy task. Influenza infections translate into a wide range of symptoms.

Some people do not develop any symptoms or only very mild ones, while others develop complications and might be hospitalized or sometimes sadly die. Historically, epidemiologists have mostly observed people seeking healthcare (at their General Practitioner’s or in hospitals), which represents a small fraction of the most severe cases.

Using mathematical models is a way to detect the real penetration of the disease in the population.

Using mathematical models is a way to detect the real penetration of the disease in the population, as ‘hidden’ cases will have an impact on the way the disease progresses and thus can be observed indirectly.

In 2009, during a pandemic, we managed to show that the figures derived from the traditional surveillance system were underestimating the actual numbers of cases. These results were confirmed shortly afterwards by a survey measuring antibodies present in the blood of people in the UK population.

Following these promising results, we published a method in 2013 to gather different data sources to reconstruct flu transmission and recover the ‘hidden’ flu cases over several seasons in the UK. We demonstrated that children were the main transmitters of the different flu strains circulating.

Other research also showed that community based surveillance (the UK FluSurvey) where people self-report about their health can complement traditional surveillance and allow scientists to understand better the transmission of flu.

### 2. The influenza viruses are constantly evolving

Influenza viruses are part of a family of viruses that are evolving at a very fast rate. In particular, the viral surface proteins (allowing the virus to enter the host cells) are constantly changing. These proteins are the main targets of the immune system to detect invasion and ultimately eliminate the virus from the body.

One of the main consequences of this continuous evolution (drift) of the virus is that influenza vaccines, which also target the same surface proteins, have to be updated every year, and that the immune system of people who have ‘learned’ about past viruses through natural infections or immunization might get re-infected.

As shown by some researchers analyzing blood samples in Southern China, adults get infected by flu about twice every ten years. This evolutionary drift is also responsible for the variation in effectiveness of the influenza vaccine from one year to another.

### 3. Herd immunity: you don’t need to vaccinate everybody

When someone gets protected following vaccination, they also protect the people that they would have infected had they have got the infection in the first place. As a result, individual vaccinations are also breaking longer chains of transmission in the community.

When a sufficient number of people are vaccinated, the virus cannot find pathways in the population to propagate. Any introduction from outside of the population will end up contained to a small outbreak.

This protection threshold occurs at a coverage dependent of the disease. For flu it is estimated that roughly half of the (total) population would need to be vaccinated to interrupt transmission.

This phenomenon is called herd immunity and is similar to other critical phenomena observed in physics. A consequence of this phenomenon is that it might be more efficient to vaccinate high transmitters (the ones most involved in chains of transmission) rather than those more at risk of developing complications but less involved in transmission. The people at risk might benefit more in some cases from an indirect rather than a direct protection.

To assess who to target between transmitters and people most at risk, models need to assess simultaneously the impact of one or the other policy. In our study just published in BMC Medicine, we showed that vaccinating children (transmitters) and then risk groups were the most cost effective vaccination strategies against flu.

### 4. We are still trying to understand exactly how flu is passed-on

Though influenza transmission has been studied for many years, the exact modes of transmission of the influenza viruses are still not totally understood. Researchers usually hypothesized on three different routes of transmission: direct contact, large respiratory droplets and aerosols (small droplets that can remains in suspension in the air after someone sneezes).

Quantifying how much of the transmission is due to one or the other modes would allow designing better Public Health recommendations. The research is complicated as the properties of the virus are also dependent on the environment (such as temperature and humidity). Researchers from Hong Kong showed recently that aerosol transmission is probably the most important mechanism of transmission.

### 5. Travelling in public transport doesn’t make you more likely to catch flu

Packed public transportation is often blamed for respiratory diseases spreading during winter times in big cities.

Packed public transportation is often blamed for respiratory diseases spreading during winter times in big cities.

Recent research using data collected by the FluSurvey team showed that contrary to the common belief, taking public transportation does not increase the risk of catching respiratory pathogens.

It seems that more prolonged contacts in households or schools are instead the main drivers of winter epidemics.

### Extending the elderly- and risk-group programme of vaccination against seasonal influenza in England and Wales: a cost-effectiveness study

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