Influenza update 2018–2019: 100 years after the great pandemic
ABSTRACT
Four influenza pandemics, starting with the historic 1918 pandemic, have killed thousands of people around the world. Vaccination, still the most important means of preventing influenza, is currently recommended yearly for all people age 6 months and older, with a goal of vaccinating 80% of all Americans and 90% of at-risk populations. Neuraminidase inhibitors are underused, and a new drug with a different mechanism of action, baloxavir marboxil, is expected to be approved soon in the United States.
KEY POINTS
- Influenza A(H7N9) is a prime candidate to cause the next influenza pandemic.
- Influenza vaccine prevents 300 to 4,000 deaths in the United States every year.
- The 2018–2019 quadrivalent influenza vaccine contains updated A(H3N2) and B/Victoria lineage components different from those in the 2017–2018 Northern Hemisphere vaccine.
- The live-attenuated influenza vaccine, which was not recommended during the 2016–2017 and 2017–2018 influenza seasons, is recommended for the 2018–2019 influenza season.
- Influenza vaccine is recommended any time during pregnancy and is associated with lower infant mortality rates.
- Overall influenza vaccination rates remain below the 80% target for all Americans and 90% for at-risk populations.
PROMOTING VACCINATION
How effective is it?
Influenza vaccine effectiveness in the 2017–2018 influenza season was 36% overall, 67% against A(H1N1), 42% against influenza B, and 25% against A(H3N2).16 It is estimated that influenza vaccine prevents 300 to 4,000 deaths annually in the United States alone.17
A 2018 Cochrane review17 concluded that vaccination reduced the incidence of influenza by about half, with 2.3% of the population contracting the flu without vaccination compared with 0.9% with vaccination (risk ratio 0.41, 95% confidence interval 0.36–0.47). The same review found that 71 healthy adults need to be vaccinated to prevent 1 from experiencing influenza, and 29 to prevent 1 influenza-like illness.
Several recent studies showed that influenza vaccine effectiveness varied based on age and influenza serotype, with higher effectiveness in people ages 5 to 17 and ages 18 to 64 than in those age 65 and older.18–20 A mathematical model of influenza transmission and vaccination in the United States determined that even relatively low-efficacy influenza vaccines can be very useful if optimally distributed across age groups.21
,Vaccination rates are low, and ‘antivaxxers’ are on the rise
Although the influenza vaccine is recommended in the United States for all people age 6 months and older regardless of the state of their health, vaccination rates remain low. In 2016, only 37% of employed adults were vaccinated. The highest rate was for government employees (45%), followed by private employees (36%), followed by the self-employed (30%).22
A national goal is to immunize 80% of all Americans and 90% of at-risk populations (which include children and the elderly).23 The number of US hospitals that require their employees to be vaccinated increased from 37.1% in 2013 to 61.4% in 2017.24 Regrettably, as of March 2018, 14 lawsuits addressing religious objections to hospital influenza vaccination mandates have been filed.25
Despite hundreds of studies demonstrating the efficacy, safety, and cost savings of influenza vaccination, the antivaccine movement has been growing in the United States and worldwide.26 All US states except West Virginia, Mississippi, and California allow nonmedical exemptions from vaccination based on religious or personal belief.27 Several US metropolitan areas represent “hot spots” for these exemptions.28 This may render such areas vulnerable to vaccine-preventable diseases, including influenza.
Herd immunity: We’re all in this together
Some argue that the potential adverse effects and the cost of vaccination outweigh the benefits, but the protective benefits of herd immunity are significant for those with comorbidities or compromised immunity.
Educating the public about herd immunity and local influenza vaccination uptake increases people’s willingness to be vaccinated.29 A key educational point is that at least 70% of a community needs to be vaccinated to prevent community outbreaks; this protects everyone, including those who do not mount a protective antibody response to influenza vaccination and those who are not vaccinated.
DOES ANNUAL VACCINATION BLUNT ITS EFFECTIVENESS?
Some studies from the 1970s and 1980s raised concern over a possible negative effect of annual influenza vaccination on vaccine effectiveness. The “antigenic distance hypothesis” holds that vaccine effectiveness is influenced by antigenic similarity between the previous season’s vaccine serotypes and the epidemic serotypes, as well as the antigenic similarity between the serotypes of the current and previous seasons.
A meta-analysis of studies from 2010 through 2015 showed significant inconsistencies in repeat vaccination effects within and between seasons and serotypes. It also showed that vaccine effectiveness may be influenced by more than 1 previous season, particularly for influenza A(H3N2), in which repeated vaccination can blunt the hemagglutinin antibody response.30
A study from Japan showed that people who needed medical attention for influenza in the previous season were at lower risk of a similar event in the current season.31 Prior-season influenza vaccination reduced current-season vaccine effectiveness only in those who did not have medically attended influenza in the prior season. This suggests that infection is more immunogenic than vaccination, but only against the serotype causing the infection and not the other serotypes included in the vaccine.
An Australian study showed that annual influenza vaccination did not decrease vaccine effectiveness against influenza-associated hospitalization. Rather, effectiveness increased by about 15% in those vaccinated in both current and previous seasons compared with those vaccinated in either season alone.32
European investigators showed that repeated seasonal influenza vaccination in the elderly prevented the need for hospitalization due to influenza A(H3N2) and B, but not A(H1N1)pdm09.33
