Flu Season: Who’s at Risk and Why

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Flu season is upon us, and many people will be getting their yearly vaccine to protect them from this potentially lethal virus. The vaccine is offered to people at risk of flu, such as over 65s, pregnant women, and young children. So why have these groups been identified as being at risk of catching flu? And how effective is it at protecting them?

The elderly are at high risk of flu – a study of French nursing homes found that 11.23% of residents caught flu during the winter. In winter, 5% of deaths among the elderly will be from flu, and the risk of a person over 80 dying of flu is 11 times higher than it is for 65-69 year olds. Elderly people are more likely to have underlying health conditions that make them susceptible to flu, like COPD or heart disease, and they have weaker immune systems than younger people. Vaccinating the elderly with virus strains that match the circulating strains has been found to reduce the number of flu cases during regional and larger outbreaks, and when there are sporadic cases in the area. Protection during outbreaks occurs even when the vaccination strains are not the same as those circulating.

Pregnant women are given the vaccine because flu can be more dangerous to them if they have an underlying medical condition, for example a cardiac or respiratory illness. There are also risks to the foetus, such as low birth weight and problems due to premature labour. Miscarriage due to maternal death from flu is also a risk. Immunisation not only protects the mother from the virus, the foetus is also protected thanks to antibodies that are passed across the placenta. The foetus retains this immunity after birth, but like everyone else, would still need to be vaccinated the next winter to protect it from that year’s strains of the virus. Vaccinating pregnant women has been found to be 63% effective in protecting their babies from flu, while reducing rates of febrile illness in the mothers by 36%. Another study found that the babies of vaccinated mothers were 45-48% less likely to be hospitalised with flu than babies whose mothers were unvaccinated. The vaccine has been shown to be safe for both the mother and the foetus, with fewer than 150 adverse effects being reported over a ten year period, and 1.9 miscarriages per 1 million women.

Children are at risk of flu for a number of reasons. They shed infectious virus particles for longer than adults do, and in higher titres. This, combined with the dense population of a school and less well developed immune systems, means that children are at an increased risk of catching and spreading flu. Flu is linked to exacerbated asthma symptoms, a disease that is common in children, which can lead to hospitalisation. Young children are more likely than adults to be hospitalised with flu, hospitalisation rates are similar to those in other high-risk groups such as the elderly and immunocompromised. Despite this, vaccination rates among children are lower than among the elderly. Unvaccinated children could be protected however, due to herd immunity. Around 80% of the population must be vaccinated in order to establish herd immunity; actual vaccination rates in the UK are lower than this, and vaccination targets are also lower than 80%. After being vaccinated, around 25% of children will develop symptoms including fever, loss of appetite, and myalgia, which persist for a few days. The benefits of vaccination far outweigh the disadvantages, as the adverse effects are typically mild and fleeting.

Vaccinating people in high-risk groups against flu has been shown to be effective in lowering infections, hospitalisations, and deaths, with minimal side effects. The levels of vaccination are not high enough to establish herd immunity however, which would protect the rest of the unvaccinated population.

References:
Keller-Stanislawski B, Englund JA, Kang G, Mangtani P, Neuzil K, Nohynek H, Pless R, Lambach P, & Zuber P (2014). Safety of immunization during pregnancy: A review of the evidence of selected inactivated and live attenuated vaccines. Vaccine, 32 (52), 7057-7064 PMID: 25285883

Thomas, R. (2014). Are influenza-associated morbidity and mortality estimates for those ≥65 in statistical databases accurate, and an appropriate test of influenza vaccine effectiveness? Vaccine, 32 (51), 6884-6901 DOI: 10.1016/j.vaccine.2014.08.090

Darvishian M, Bijlsma MJ, Hak E, & van den Heuvel ER (2014). Effectiveness of seasonal influenza vaccine in community-dwelling elderly people: a meta-analysis of test-negative design case-control studies. The Lancet. Infectious diseases, 14 (12), 1228-39 PMID: 25455990

Brady RC, Hu W, Houchin VG, Eder FS, Jackson KC, Hartel GF, Sawlwin DC, Albano FR, & Greenberg M (2014). Randomized trial to compare the safety and immunogenicity of CSL Limited’s 2009 trivalent inactivated influenza vaccine to an established vaccine in United States children. Vaccine, 32 (52), 7141-7147 PMID: 25454878

Cabeça TK, Watanabe A, Moreira LP, Melchior TB, Perosa AH, Camargo C, Parmezan SN, & Bellei N (2014). Influenza virus surveillance among young children in São Paulo, Brazil: The impact of vaccination. Brazilian journal of microbiology : [publication of the Brazilian Society for Microbiology], 45 (3), 1113-5 PMID: 25477951

Photo:
PAHO/WHO via photopin cc

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