Influenza vaccine

Also known as: Flu vaccine

Definition

The influenza vaccine helps to protect against infection with the influenza virus. Influenza is an acute viral respiratory illness with abrupt onset and is spread primarily by respiratory droplets from person to person (mainly through inhalation of virus-containing droplets). Influenza is caused by a group of viruses of the Orthomyxoviridae family, which are separated into three strain types (A, B, and C) according to their nuclear material.

Vaccination is the most effective protection against influenza. The vaccine may be administered to anyone over six months old wishing to reduce the risk of influenza, and the US Centers for Disease Control (CDC) suggests universal vaccination in order to provide the best possible protection. It is recommended especially for unhealthy persons and for persons who are likely to transmit influenza to unhealthy persons in a given community. However, some people should not receive the vaccine due to other conditions. Anyone with life-threatening allergies to any element of the vaccine should avoid vaccination or talk to their doctor about what is suitable. People with egg allergies can usually receive the vaccine but should consult their doctor, especially if they have had reactions other than hives to eggs. Those with a history of Guillain-Barré Syndrome (GBS) may also be advised not to receive the flu vaccine. Finally, people who are already sick should consider waiting to be vaccinated when they are feeling better.

Influenza vaccines are designed to trigger an immune response to hemagglutinin and neuraminidase, the two proteins found on the surface of the influenza virus. These proteins are always changing (mutating), so every year, seasonal influenza vaccines have to be reformulated with the three strains that are likely to be more effective in fighting new influenza strains.

The World Health Organization’s Global Influenza Programme monitors the influenza viruses circulating among humans worldwide and quickly identifies new strains so that new, appropriate vaccines can be made for a particular year.

Types of Influenza Vaccines

The trivalent inactivated influenza vaccine (TIV) has been available since the mid-twentieth century. TIV is administered by intramuscular or intradermal routes and contains three inactivated viruses: type A (H1N1), type A (H3N2), and type B. The influenza vaccine viruses are grown in chicken eggs; thus, the final product contains residual egg protein. The vaccines are also available in both pediatric- and adult-dose formulations and can be preservative-free in a single vial or in a multidose vial with thimerosal as a preservative.

The live attenuated influenza vaccine (LAIV), which contains the same three influenza viruses as TIV, is administered by intranasal route. LAIV viruses are also grown in chicken eggs. LAIV is preservative-free and is provided in a single-dose sprayer unit with one-half the dosage sprayed into each nostril. LAIV is not recommended to be used for some flu seasons, depending on the severity of the strain and the effectiveness of the vaccine.

Improved technology and innovation have enabled improved methods of administering influenza vaccines, including a reduced-dose injectable made possible by the addition of adjuvants and the use of a cell culture vaccine. Scientists are also exploring new routes of administration, such as intradermal (with or without needle) and transcutaneous, in which a patch delivers the vaccine through micro-needles that may barely penetrate the skin before dissolving and releasing the vaccine.

Efficacy and Further Research

Most vaccinated persons develop postvaccination hemagglutination inhibition antibody titers. These antibodies are protective against illness caused by strains similar to those in the vaccine or by related variants that may emerge during outbreaks. However, while vaccines are considered the best protection against influenza, they are not 100 percent effective. The constantly mutating nature of the influenza virus means that vaccine makers are continually trying to keep up with the latest strain, and the prevalent method of growing the vaccine in eggs means that the vaccine itself may be prone to mutate into a less effective form.

Even the most successful vaccines usually only protect about 60 percent of those receiving them, and in years when the vaccine is poorly matched to the prevalent strain, the efficacy can be as low as 10 percent protection. For example, during the 2021-2022 flu season, the influenza vaccine was only 36 percent effective against influenza A. However, the 2022-2023 flu season vaccine has proved to be a better match for the prevalent strains reducing hospitalizations by 75 percent in children and 50 percent in adults. Vaccination is further complicated by the complexity and unique character of individual immune systems, which can interact differently with even vaccines made from the correct seasonal strain.

Amid debates over the effectiveness of vaccinations, concern had also always existed over the high number of elderly adults who would still succumb to influenza and possibly lose their lives despite having been vaccinated because of their naturally weaker immune systems. Therefore, in late 2015, the US Food and Drug Administration (FDA) approved an influenza vaccine booster that had previously been in use in several other countries. The vaccine with the adjuvant, which helps stimulate the immune system to make the vaccine more effective, is known as Fluad and was first made available in the United States in 2016.

Additionally, scientists have continued to experiment on whether a universal vaccine could be produced that would help the immune system fight groups of viruses rather than a specific strain. Research suggests that new methods of vaccine development, such as genetic engineering instead of cultivation in eggs, may help improve vaccine efficacy. However, it has been suggested that many scientists are not inclined to openly discuss issues with existing vaccines due to fears of stoking the arguments of the anti-vaccination movement, which is disregarded as pseudoscience.

Bibliography

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