Primary prevention

Primary prevention for influenza is provided by the influenza vaccine.[38][39][40][41][42][43][44][45] [ Cochrane Clinical Answers logo ] [ Cochrane Clinical Answers logo ] [ Cochrane Clinical Answers logo ]  International guidelines vary in their recommendations of who should receive vaccination. The US Centers for Disease Control and Prevention (CDC) currently recommends influenza vaccine for all people aged ≥6 months who do not have contraindications to vaccination.[46] CDC: influenza (flu) external link opens in a new window For 2019-2020, the CDC Advisory Committee on Immunization Practices (ACIP) recommend that any licensed, age-appropriate influenza vaccine may be used, including the live attenuated vaccine where it is appropriate.[46] The American Academy of Pediatrics (AAP) also now states that both inactivated influenza and live attenuated influenza vaccine are options for children, with no preference (unless contraindicated).[14] Trials are under way for use of the influenza vaccine in young infants (<6 months of age).[47] Children aged 6 months to 8 years require two doses of influenza vaccine (administered ≥4 weeks apart) during their first season of vaccination to optimise response.[46] The ACIP recommends that children aged 6 months to 8 years who have previously received ≥2 doses of trivalent or quadrivalent influenza vaccine before July 1, 2019, require only one dose for 2019-2020.[46] Beginning at age 9 years, only one annual dose is recommended.[48] Pregnant women may receive any licensed, recommended, age-appropriate inactivated vaccine; the live attenuated influenza vaccine should not be used during pregnancy.[46] Influenza vaccination during pregnancy is not associated with an increased risk of infant hospitalisation or death in the first 6 months of life,[49] and may help prevent influenza hospitalisations among pregnant women.[50] Additionally, influenza vaccination during pregnancy results in antibody development that can protect infants in the first few months of life.[51] Other international guidelines recommend vaccination for high-risk groups. Local guidelines should be consulted and followed.

For the 2019-2020 (northern hemisphere) season, the World Health Organization recommends that quadrivalent vaccines contain an A/Brisbane/02/2018 (H1N1)pdm09-like virus, an A/Kansas/14/2017 (H3N2)-like virus, a B/Colorado/06/2017-like virus (B/Victoria/2/87 lineage), and a B/Phuket/3073/2013-like virus (B/Yamagata/16/88 lineage). The influenza B component of trivalent vaccines should be a B/Colorado/06/2017-like virus of the B/Victoria/2/87-lineage.[52]

In the US, the AAP recommends all children with egg allergy of any severity can receive either an inactivated influenza vaccine or live attenuated influenza vaccine without any additional precautions beyond those recommended for any vaccine.[14] The ACIP recommends the following:[46][53]

  • People with a history of egg allergy who have experienced only hives after exposure to egg should receive influenza vaccine (any licensed, recommended, age-appropriate influenza vaccine).

  • People who have reported reactions to egg (involving such symptoms as angio-oedema, respiratory distress, lightheadedness, or recurrent emesis) or who required epinephrine (adrenaline) or another emergency medical intervention may receive influenza vaccine (any licensed, recommended, age-appropriate influenza vaccine). The vaccine should given by a physician with experience in the recognition and management of severe allergic conditions.

  • People who are able to eat lightly cooked egg (e.g., scrambled egg) without reaction are unlikely to be allergic. Egg-allergic people might tolerate egg in baked products (e.g., bread or cake). Tolerance to egg-containing foods does not exclude the possibility of egg allergy. Egg allergy can be confirmed by a consistent medical history of adverse reactions to eggs and egg-containing foods, plus skin and/or blood testing for immunoglobulin E directed against egg proteins.

  • For people with no known history of exposure to egg, but who are suspected of being egg-allergic on the basis of previously performed allergy testing, consultation with a physician with expertise in the management of allergic conditions should be obtained before vaccination.

  • A previous severe allergic reaction to influenza vaccine, regardless of the component suspected of being responsible for the reaction, is a contraindication to future receipt of the vaccine.

Regardless of allergy history, all vaccines should be administered in settings in which personnel and equipment for rapid recognition and treatment of anaphylaxis are available.

Administering the vaccine by a two-step approach and skin testing with vaccine are not necessary.[54] A trivalent vaccine made using an insect virus expression system and recombinant DNA technology is now available for adults aged 18 years or older. Unlike other vaccines, this vaccine does not use the influenza virus or eggs in the manufacturing process. A cell-based quadrivalent vaccine is available in the US for people aged ≥4 years, and in Europe for people aged ≥9 years. The cell-based quadrivalent vaccine is made using influenza viruses grown in cultured cells of mammalian origin instead of hen’s eggs.[55][56]

Vaccine effectiveness may vary according to the age of the recipient, the level of pre-existing immunity, and correctly predicting the specific circulating strains of virus.[57][40][41][42][43][58][59][60][61][62][63][64][65] Evidence A

Vaccine efficacy in older people:

  • Most influenza-associated deaths occur in the older population (aged ≥65 years).[66][67][68] A systematic review found that benefits of vaccination were more evident on health-related outcomes of residents in long-term care facilities than in healthy older individuals in the community.[69] A pooled cohort study published after the meta-analyses demonstrated a significant reduction in mortality in vaccinated older individuals (1.0% versus 1.6% in unvaccinated individuals).[67] The mortality benefit in older patients is increased with annual vaccination.[70] High-dose vaccination and boosting low titres midseason may be of benefit in older people.[71][72][73][74][75]

  • Any licensed, age-appropriate vaccine may be used.[46]

Guillain-Barre syndrome (GBS) risk:

  • GBS is an acute autoimmune disorder of peripheral nerves that develops in susceptible individuals after infection and, in rare cases, after immunisation. In the US, an increased risk of GBS was associated with the 1976 swine influenza vaccine (swine-origin influenza A H1N1 subtype A/NJ/76).[76] The number of reports of influenza-vaccine-associated GBS to the national Vaccine Adverse Event Reporting System increased from 37 in 1992-1993 to 74 in 1993-1994. Studies of these cases showed that for the two seasons combined, the adjusted relative risk of 1.7 suggests slightly more than 1 additional case of GBS per million people vaccinated against influenza.[77] This risk seems to be substantially less than the overall health risk posed by naturally occurring influenza.

  • A history of GBS within 6 weeks following receipt of influenza vaccine is a precaution for the use of influenza vaccine because of the risk of recurrent GBS. Risks and benefits of vaccination need to be considered in these instances.[46]

Secondary prevention

Both oseltamivir and zanamivir have been shown to be effective as prophylaxis against infection when given early after exposure to an infected individual.[135] [ Cochrane Clinical Answers logo ] Antiviral prophylaxis after exposure to an infected individual is reserved for at-risk populations.[109][110] Findings from a meta-analysis showed that oseltamivir modestly reduces time to clinical symptom alleviation in adults with influenza, but increased the incidence of nausea and vomiting.[125]

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