The influenza virus began circulating early in 2022, giving an earlier start to the season and vaccination expectations. This epidemiological scenario had been predicted, since the same phenomenon occurred in regions of the southern hemisphere.
In the United States, vaccination against influenza A and B viruses is recommended for everyone six months and older, with no contraindications. 
Nine vaccines are currently available on the U.S. market, according to a report published by The Medical Letter on November 28:
- Standard-dose, egg-grown inactivated quadrivalent (IIV4). Afluria Quadrivalent (Seqirus); Fluarix Quadrivalent (GSK); FluLaval Quadrivalent (GSK); Fluzone Quadrivalent (Sanofi).
- High-dose, egg-grown inactivated quadrivalent (HD-IIV4). Fluzone High-Dose Quadrivalent (Sanofi).
- Standard-dose, egg-grown adjuvanted inactivated quadrivalent (aIIV4). Fluad Quadrivalent (Seqirus).
- Standard-dose, cell culture-based inactivated quadrivalent (ccIIV4). Flucelvax Quadrivalent (Seqirus).
- Recombinant quadrivalent (RIV4). Flublok Quadrivalent (Sanofi).
- Live attenuated, egg-grown quadrivalent (LAIV4). FluMist Quadrivalent (AstraZeneca).
Influenza vaccination reduces the incidence of laboratory-confirmed virus and the risk of severe complications and death associated with the disease. The effectiveness of this seasonal influenza vaccine in preventing influenza disease depends on several factors, including the match between the vaccine and circulating strains, and the recipient’s immune response.
Vaccine effectiveness is greatest when there is a strong match, but even when the match is suboptimal, vaccination can still substantially reduce the risk of influenza-related hospitalization and death.
This is true even among the populations who are most vulnerable to the virus, such as older adults, and particularly older adults who have had cardiovascular episodes prior to infection.
For this demographic, the U.S. Food and Drug Administration (FDA) has approved the high-dose quadrivalent vaccines, which contain four times as many antigens as a standard vaccine.
In several studies with older adults, use of a high-dose inactivated trivalent vaccine was associated with a reduced risk of respiratory-related and all-cause hospitalization and death, compared with standard-dose inactivated trivalent vaccines. The quadrivalent version goes a step further in the challenge of vaccinating against influenza, which causes between 290,000 and 650,000 deaths worldwide each year.
In Latin America and the Caribbean, influenza deaths total about 72,000 each year, while in the United States deaths range from 12,000 to 52,000, depending on the season.
Another vaccine intended for seniors 65 and older is the adjuvanted quadrivalent vaccine. This is an oil-in-water emulsion of squalene oil that enhances the immune response by “recruiting” antigen-presenting cells to the injection site and promoting the uptake of influenza virus antigens.
In a randomized trial with 7,082 adults aged 65 years and older, an adjuvanted inactivated trivalent vaccine elicited significantly greater antibody responses against all three influenza strains than a nonadjuvanted inactivated trivalent vaccine.
In several trials, older adults who received an adjuvanted inactivated trivalent vaccine were less likely to develop symptomatic influenza or to be hospitalized for influenza or pneumonia than those who received a nonadjuvanted trivalent inactivated vaccine.
The recombinant quadrivalent vaccine, which is not grown in eggs and does not contain forms of the influenza virus, contains three times more antigens than standard vaccines. It is approved for use in persons 18 years of age and older.
In a randomized, double-blind trial of 8,604 adult participants aged 50 and older during the A/H3N2-predominant 2014-2015 influenza season, the recombinant quadrivalent vaccine was 30% more effective than a nonadjuvanted standard-dose inactivated quadrivalent vaccine in preventing laboratory-confirmed influenza infection.
For pregnant women, the Advisory Committee on Immunization Practices (ACIP) and the American College of Obstetricians and Gynecologists (ACOG) recommend influenza vaccination to protect against influenza infection, which can be severe during pregnancy. Pregnant women may receive any age-appropriate version of the vaccine. However, experts do not recommend the use of the intranasal live attenuated vaccine (FluMist Quadrivalent).
With regard to egg-grown vaccines, the recombinant vaccine (Flublok Quadrivalent) and the cell culture-based inactivated vaccine (Flucelvax Quadrivalent) do not contain egg protein.
Other available influenza vaccines may contain traces of egg protein (ovalbumin), but numerous studies have found that patients with a history of egg allergy are not at increased risk for a reaction to egg-grown influenza vaccines.
The ACIP states that people with a history of various levels of egg allergy can receive any age-appropriate influenza vaccine. However, those with a history of severe egg allergy who receive an egg-based vaccine should be vaccinated in a medical setting, supervised by a health care provider with experience in managing severe allergic reactions.
For pediatric influenza vaccines, the American Association of Pediatrics says that inquiring about egg allergy is not required before administering any of the influenza vaccines.
Immunocompromised persons should not receive live attenuated influenza vaccines. Inactivated and recombinant vaccines are generally considered safe, but the immune response may be lower than in the general population. In two randomized trials in solid organ transplant recipients, the high-dose vaccine induced significantly greater immune responses than standard-dose vaccines.
The next goal of scientific research in the field of influenza vaccination is the development of a universal vaccine that would protect against all strains of the virus. Research is also underway for a vaccine that generates immune protection against the influenza virus, COVID-19, and respiratory syncytial virus (RSV).
This story was produced using content from the original study and from other medical research, as well as health and public health sources, highlighted in related links throughout the article.
