The 2018–2019 quadrivalent vaccine for the Northern Hemisphere1 contains the following strains:
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The A(H3N2) (Singapore) and B/Victoria lineage components are new this year. The A(H3N2) strain was the main cause of the 2018 influenza epidemic in the Southern Hemisphere.
The quadrivalent live-attenuated vaccine, which was not recommended during the 2016–2017 and 2017–2018 influenza seasons, has made a comeback and is recommended for the 2018–2019 season in people for whom it is appropriate based on age and comorbidities.2 Although it was effective against influenza B and A(H3N2) viruses, it was less effective against the influenza A(H1N1)pdm09-like viruses during the 2013–2014 and 2015–2016 seasons.
A/Slovenia/2903/2015, the new A(H1N1)pdm09-like virus included in the 2018–2019 quadrivalent live-attenuated vaccine, is significantly more immunogenic than its predecessor, A/Bolivia/559/2013, but its clinical effectiveness remains to be seen.
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).3 It is estimated that influenza vaccine prevents 300 to 4,000 deaths annually in the United States alone.4
A 2018 Cochrane review4 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.5-7 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.8
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%).9
A national goal is to immunize 80% of all Americans and 90% of at-risk populations (which include children and the elderly).10 The number of US hospitals that require their employees to be vaccinated increased from 37.1% in 2013 to 61.4% in 2017.11 Regrettably, as of March 2018, 14 lawsuits addressing religious objections to hospital influenza vaccination mandates have been filed.12
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.13 All US states except West Virginia, Mississippi and California allow nonmedical exemptions from vaccination based on religious or personal belief.14 Several US metropolitan areas represent “hot spots” for these exemptions.15 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.16 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.
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.17
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.18 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.19
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)pdm09.20
High-dose vaccine for older adults. The high-dose influenza vaccine has been licensed since 2009 for use in the United States for people ages 65 and older. Recent studies confirmed that high-dose vaccine is more effective than standard-dose vaccine in veterans21 and US Medicare beneficiaries.22
The high-dose vaccine is rapidly becoming the primary vaccine given to people ages 65 and older in retail pharmacies, where vaccination begins earlier in the season than in providers’ offices.23 Some studies have shown that the standard-dose vaccine wanes in effectiveness toward the end of the influenza season (particularly if the season is long) if it is given very early. It remains to be seen whether the same applies to the high-dose influenza vaccine.
Some advocate twice-annual influenza vaccination, particularly for older adults living in tropical and subtropical areas, where influenza seasons are more prolonged. However, a recently published study observed reductions in influenza-specific hemagglutination inhibition and cell-mediated immunity after twice annual vaccination.24
Vaccination is beneficial during pregnancy. Many studies have shown the value of influenza vaccination during pregnancy for both mothers and their infants. One recently published study showed that 18% of infants who developed influenza required hospitalization.25 In that study, prenatal and postpartum maternal influenza vaccination decreased the odds of influenza in infants by 61% and 53%, respectively.
Another study showed that vaccine effectiveness did not vary by gestational age at vaccination.26
Some studies have shown that influenza virus infection can increase susceptibility to certain bacterial infections. A post hoc analysis of an influenza vaccination study in pregnant women suggested that the vaccine was also associated with decreased rates of pertussis in these women.27
Several factors including age-related frailty and iatrogenic and disease-related immunosuppression can affect vaccine effectiveness.
Frailty. A recent study showed that vaccine effectiveness was 77.6% in nonfrail older adults but only 58.7% in frail older adults.28
Immunosuppression. Temporary discontinuation of methotrexate for 2 weeks after influenza vaccination in patients with rheumatoid arthritis improves vaccine immunogenicity without precipitating disease flare.29 Solid-organ and hematopoietic stem cell transplant recipients who received influenza vaccine were less likely to develop pneumonia and require intensive care unit admission.30
The high-dose influenza vaccine is more immunogenic than the standard-dose vaccine in solid-organ transplant recipients.31
Statins are widely prescribed and have recently been associated with reduced influenza vaccine effectiveness against medically attended acute respiratory illness, but their benefits in preventing cardiovascular events outweigh this risk.32
Moving away from eggs. During the annual egg-based production process, which takes several months, the influenza vaccine acquires antigenic changes that allow replication in eggs, particularly in the hemagglutinin protein, which mediates receptor binding. This process of egg adaptation may cause antigenic changes that decrease vaccine effectiveness against circulating viruses.
The cell-based baculovirus influenza vaccine grown in dog kidney cells has higher antigenic content and is not subject to the limitations of egg-based vaccine, although it still requires annual updates. A recombinant influenza vaccine reduces the probability of influenza-like illness by 30% compared with the egg-based influenza vaccine, but also still requires annual updates.33 The market share of these non-egg-based vaccines is small, and thus their effectiveness has yet to be demonstrated.
The US Department of Defense administered the cell-based influenza vaccine to about one-third of Armed Forces personnel, their families, and retirees in the 2017–2018 influenza seasons, and data on its effectiveness are expected in the near future.34
A universal vaccine would be ideal. The quest continues for a universal influenza vaccine, one that remains protective for several years and does not require annual updates.35 Such a vaccine would protect against seasonal epidemic influenza drift variants and pandemic strains. More people could likely be persuaded to be vaccinated once rather than every year.
The structure of the influenza virus can be depicted as a ball with spikes representing the hemagglutinin protein on its surface, each spike in the form of a stalk and a cap, as shown in the figure below.36,37 All current influenza vaccines aim at the cap portion of the hemagglutinin protein.
Annual antigenic drift of influenza viruses alters the cap portion of the hemagglutinin protein, requiring annual vaccine updates. The stalk portion of the hemagglutinin protein is consistent among different influenza viruses and is not altered annually like the cap portion. Thus, a vaccine aimed at the stalk portion of the hemagglutinin protein has the potential to be a universal vaccine.
An ideal universal vaccine would be suitable for all age groups, at least 75% effective against symptomatic influenza virus infection, protective against all influenza A viruses (influenza A, not B, causes pandemics and seasonal epidemics), and durable through multiple influenza seasons.38
Research and production of such a vaccine are expected to require funding of about $1 billion over the next 5 years.
Boosting effectiveness. Estimates of influenza vaccine effectiveness range from 40% to 60% in years when the vaccine viruses closely match the circulating viruses, and variably lower when they do not match. The efficacy of most other vaccines given to prevent other infections is much higher.
New technologies to improve influenza vaccine effectiveness are needed, particularly for influenza A(H3N2) viruses, which are rapidly evolving and are highly susceptible to egg-adaptive mutations in the manufacturing process.
In one study, a nanoparticle vaccine formulated with a saponin-based adjuvant induced hemagglutination inhibition responses that were even greater than those induced by the high-dose vaccine.39
Immunoglobulin A (IgA) may be a more effective vaccine target than traditional influenza vaccines that target IgG, since different parts of IgA may engage the influenza virus simultaneously.40
Vaccines can be developed more quickly than in the past. The timeline from viral sequencing to human studies with deoxyribonucleic acid plasmid vaccines decreased from 20 months in 2003 for the severe acquired respiratory syndrome coronavirus to 11 months in 2006 for influenza A/Indonesia/2006 (H5), to 4 months in 2009 for influenza A/California/2009 (H1), to 3.5 months in 2016 for Zika virus.41 This is because it is possible today to sequence a virus and insert the genetic material into a vaccine platform without ever having to grow the virus.
This post is a shortened adaptation of a review article published in the November 2018 issue of Cleveland Clinic Journal of Medicine (2018;85:861-869).