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Young people are thought to drive the spread of seasonal influenza (flu) while rarely becoming seriously ill themselves. A computer model developed before the pandemic showed the benefits of including teenagers in the vaccination programme. The model found that giving flu vaccines to 2 – 20 year olds cost less than vaccinating people over 65 and prevented a higher burden of disease in the population. It found that this approach reduced the spread of the virus and protected those who do not have immunity.

Before the pandemic, children aged 2 – 11 years were routinely offered vaccination in order to reduce the spread of flu.  This was along with adults who are at risk of becoming seriously ill with flu because they are over 65, pregnant or have a long-term condition such as asthma, heart disease or diabetes. 

This flu season could be exceptional with more people than usual catching flu because social distancing has meant they have not built up natural immunity. In addition, anyone who has flu and COVID-19 at the same time is more likely to become seriously ill. As a temporary measure therefore, young people aged 12 - 16 are being included in the programme this winter. People aged 50 - 65 years are also now included.

Unlike previous models, this model included data on people over a number of flu seasons. This means it took into account the immunity that people built up in previous years. The research suggests that vaccinating 11 – 20 year olds is a good use of healthcare resources. 

Flu vaccines are given every year and the programme is expensive. NHS policymakers have to decide how to use finite resources in the most cost-effective way. Findings from this study are being used to inform flu vaccination policy. 

Further information on flu vaccination is available on the NHS website.

What’s the issue?

In the UK, flu outbreaks occur most often in the winter, peaking between January and March. Typical symptoms are high temperature, aches and fatigue but the illness can result in life-threatening complications such as pneumonia. Before the pandemic, flu caused one in ten hospital admissions for respiratory problems and deaths in the UK. Most people who die from flu are over the age of 65.

New strains of flu are constantly emerging. A new vaccine is produced every year to protect people from the dominant strain. The NHS offers free vaccination to people at high risk of becoming seriously ill with flu. Before 2020, this included people over 65, pregnant women, people in long-term residential care, and those with some health conditions. Flu vaccination has been offered to children aged between 2 – 11 years since 2013. 

In response to the COVID-19 pandemic, the over 50s and children aged 11 – 16 years are currently being offered a flu vaccine. The aim is to reduce the burden on the NHS at a time when hospitals continue to deal with COVID-19 patients. But vaccinating huge numbers of people every year is costly and time-consuming for the NHS. Policymakers must decide how to protect the population against flu in the most cost-effective way. 

Computer models based on health data can help work out the best vaccination strategy. But most previous models have considered each flu season and each strain separately. This means they could not take account of how infection or vaccination in previous years changes a person’s risk in the future. 

By contrast, this model includes data about immunity built up in previous flu seasons and looks at its impact in later years. 

What’s new?

The model was developed as part of the Mathematical and Economic Modelling for Vaccination and Immunisation Evaluation (MEMVIE) project. Researchers reconstructed the way that flu spread in several flu seasons (2009 – 2018) by including information about peoples’ infection and vaccination history. 

They used data which represented all sectors of the population in England. It came from databases on GP consultations, national reports on flu (from Public Health England, now known as UK Health Security Agency) and information on the circulating strain (from FluNet).

The research team used their model to explore what would happen if flu vaccination was offered to different groups of people. They looked at 1,375 different vaccination strategies, offering vaccines to various age groups. They estimated the effect of vaccinating younger people at low risk of complications, as well as to those at high risk. They compared each strategy to only vaccinating people at risk. 

Using the model, they assessed the cost-effectiveness of extending the flu vaccination programme. They considered the effect of vaccinating different age groups who were at low risk of developing complications from flu.  

The results suggested that vaccinating low-risk children and young people aged 2 - 20 is a highly cost-effective way of tackling seasonal flu. Vaccinating healthy people over 65 (the current approach) is cost-effective, but vaccinating 2 – 20 year olds, even more so.

The research shows the benefits of vaccinating the under 20s in addition to those at risk (people over 65, pregnant women, people in long-term residential care, and those with some health conditions).

Why is this important?

Because of the pandemic, the flu vaccination programme has been expanded to include young people aged 12 - 16, and older people aged 50 - 65 years. This study was carried out before the pandemic but it supports the move to vaccinate teenagers. It found that vaccinating children and young people aged 2 – 20 costs less than vaccinating people over 65 and may prevent a higher burden of disease in the population.  

This is probably because young people drive flu transmission. Young adults tend to have contact with a broad range and large number of people whom they could transmit a flu infection to. Some young adults are also parents of young children and are at risk of contracting illnesses from close contact with them. 

Vaccinating low-risk children and young people aged 2 – 20 (in addition to people at risk) is likely to contribute to herd immunity across the whole population. Herd immunity occurs when enough people are immune to an infection to curb its spread. This fall in transmission then protects people who are not immune. 

What’s next?

Findings from this study are being used to inform flu vaccination policy in the UK. The research team worked in collaboration with the Department of Health and Social Care to ensure their modelling met their needs. The research has also been used by the Joint Committee on Vaccination and Immunisation (JCVI) in their discussions on flu vaccination. 

Social distancing measures to address the COVID-19 pandemic curbed the circulation of flu in 2020. As flu infections have been so low, peoples’ immunity to flu may have waned. As a result, this winter’s flu season (2021-22) is likely to be more severe than usual. More research is needed to understand how flu and COVID-19 infections impact upon each other. 

You may be interested to read

This NIHR Alert is based on: Hill EM, and others. Optimising age coverage of seasonal influenza vaccination in England: A mathematical and health economic evaluation. PLoS Computational Biology 2020;16:10 

Government guidance on who should have the flu vaccine in 2021 and why

The Mathematical & Economic Modelling for Vaccination and Immunisation Evaluation (MEMVIE) project

 

Funding: This research was funded by the NIHR Policy Research Programme, Infectious Disease Dynamic Modelling in Health Protection. 

Conflicts of Interest: One author has received funding from various pharmaceutical companies.  

Disclaimer: NIHR Alerts are not a substitute for professional medical advice. They provide information about research which is funded or supported by the NIHR. Please note that views expressed in NIHR Alerts are those of the author(s) and reviewer(s) and not necessarily those of the NHS, the NIHR or the Department of Health and Social Care.


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Study author

In future, we would like to explore how recent disruptions in flu circulation and infection due to COVID-19 lockdowns affect the different vaccination strategies simulated by our model.

There has been very low flu activity globally in the past 18 months, which may impact the amount of immunity in the population to flu viruses in circulation in future years. Here in the UK, it is highly likely that we will see a more severe influenza peak than we would otherwise have expected in the upcoming flu season.

There is a great amount of uncertainty about SARS-CoV-2 (the virus causing COVID-19) and how it interacts with other respiratory infections such as flu. Is infection by SARS-CoV-2 blocking the spread of other viruses? Or can infection with either SARS-CoV-2 or flu make it more likely a person is infected with the other virus? Our ideas and understanding will continue to be modified as new information on this outbreak is accumulated.

Edward Hill, Research Fellow, Zeeman Institute for Systems Biology & Infectious Disease Epidemiology Research, University of Warwick 

Researcher 

This study improves on previous models of seasonal influenza vaccination that have informed policy; it takes account of immunity that could build up between each influenza season. All models come with a level of uncertainty. This one follows the English population for a small number of seasons and, ideally, a model would both follow the population for longer and take genetic changes in influenza strains into account. But that would require greater scientific understanding of how this occurs and a far more complex model.

The findings from this paper confirm that the current paediatric vaccination programme in England saves lives and is likely to be a good investment. This should encourage public health and healthcare professionals, children, and their caregivers, to continue improving uptake of the programme.

Mark Jit, Professor of Vaccine Epidemiology, London School of Hygiene and Tropical Medicine 

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