How the Influenza Vaccine Works

Influenza is an acute respiratory infection which can be asymptomatic or symptomatic with sudden onset of symptoms such as fever, a dry cough, muscle pains, headache, and sore throat. In most cases the illness is self-limiting but in certain risk groups it can be severe and even fatal, write Drs. Lois O’Connor EPIET Fellow, Joan O’Donnell, Specialist in Public Health Medicine, HSE Health Protection Surveillance Centre and Kevin Kelleher, Director, Health & Wellbeing – Public Health and Child Health.

Dr. Kevin Kelleher
Dr. Kevin Kelleher

Risk groups include those aged 65 years and older, those in congregant settings such as long-term care facilities, pregnant women and those with chronic disease. Influenza-associated complications include; pneumonia, both viral and bacterial, myocarditis, encephalitis and worsening of underlying conditions such as chronic obstructive pulmonary disease, congestive heart failure and asthma. These complications may require hospitalisation and can be life threatening especially in those aged 65 years and over and in those with underlying medical conditions. Pregnant women have also been found to be at increased risk of the complications of flu.

Internationally it is estimated that influenza accounts for three to five million serious illnesses and between 250,000 and 500,000 deaths each year. In Ireland, in the 2015/2016 season almost 2000 people were hospitalised due to influenza, 161 required admission to an intensive care unit and there were 84 deaths among notified influenza cases. In addition in the 2014/2015 influenza season when influenza A(H3N2) was the predominant virus in circulation, the estimated excess deaths in those aged 65 years and older were 726.

In Ireland, in the 2015/2016 season almost 2000 people were hospitalised due to influenza, 161 required admission to an intensive care unit and there were 84 deaths among notified influenza cases

Influenza Viruses

Dr. Lois O’Connor EPIET Fellow
Dr. Lois O’Connor

Influenza virus is an RNA virus. Globally and nationally, clinical and virological surveillance of influenza are undertaken on influenza A and B viruses. Type A causes moderate to severe disease, affects all age groups and has human and animal/bird reservoirs. Type B causes milder illness, predominantly affects children but can affect any age group, and humans are the only reservoir.

Influenza A viruses have two surface antigens hemagglutinin (H) and neuraminidase (N). The viruses are divided into subtypes based on their H and N content. Mutations (changes) of the surface antigens occur regularly. Minor mutations occur from season to season, and are the reason why the vaccine composition changes each year. This is called “antigenic drift”. Major changes occur periodically and result in an immunologically distinct virus, facilitating pandemic spread with the potential for severe morbidity and high mortality. This is called “antigenic shift”. The most recent pandemic was in 2009/ 2010, caused by the Influenza A (H1N1)pdm09 virus.

In the northern hemisphere the influenza season occurs every year between October and May. In general influenza circulates in the community for an average of nine weeks and usually peaks between January and March each year. Outbreaks of influenza in residential care facilities can occur well into April.

Influenza A(H3N2) is currently predominating in Ireland and throughout most of Europe for the 2016/2017 season to date

Dr. Joan O’Donnell, Specialist in Public Health Medicine, HSE Health Protection Surveillance Centre
Dr. Joan O’Donnell

During this period influenza viruses i.e. influenza A (H1N1)pdm09, influenza A (H3N2) and influenza B circulate. Influenza A(H3N2) is currently predominating in Ireland and throughout most of Europe for the 2016/2017 season to date, with those aged 65 years and older most affected. Seasons when influenza A(H3N2) is predominant tend to affect older age groups. The number of outbreaks in residential care facilities, hospitalisations and excess all cause and influenza and pneumonia-related deaths are usually higher during influenza A(H3N2) predominant seasons.

During influenza A(H1)pdm09 predominant seasons, influenza usually affects younger age groups and results in a more severe burden on critical care units than influenza A(H3) predominant seasons.

Influenza B viruses generally cause less severe illness than type A viruses but it can also contribute significantly to the critical care unit burden and influenza B deaths are also reported. Previously type B viruses have resulted in late season sporadic outbreaks in residential care facilities. Mutations rarely occur in influenza B viruses.

Influenza Vaccine

Influenza vaccine is produced every year based on World Health Organization recommendations.

Influenza vaccination has also been shown to reduce hospitalisations by 79% in patients with diabetes and by 52% in patients with chronic lung disease

The World Health Organization (WHO) convenes technical consultations with the WHO Collaborating Centres (CCs) on Influenza and Essential Regulatory Laboratories (ERLs) of the WHO Global Influenza Surveillance and Response System in the USA, UK, Australia, China and Japan in February and September each year to recommend viruses for inclusion in influenza vaccines for the northern and southern hemisphere influenza season, respectively. The choice of strain included in the vaccine is dependent on the most frequently detected strains in the previous influenza season in that hemisphere. In addition, for the northern hemisphere vaccine, consideration is given to the viruses circulating in the southern hemisphere during their previous season.

In Ireland the trivalent influenza vaccine is used and comprises antigens from two type A and one type B virus strains. The trivalent vaccine used in Ireland for the 2016/2017 flu season is composed of antigens from the following strains:

  • an A/California/7/2009 (H1N1)pdm09-like virus;
  • an A/Hong Kong/4801/2014 (H3N2)-like virus;
  • a B/Brisbane/60/2008-like virus.

Influenza vaccine is recommended for the following groups of the population in Ireland (1)

  1. Those aged >6months of age, who are at increased risk of complications from influenza infection including the following groups: people aged 50 and older, people with chronic diseases requiring regular medical follow-up, people who are immunosuppressed, people with Down Syndrome, children with neurodevelopmental disorders, children on long-term aspirin therapy, people who are morbidly obese i.e. body mass index (BMI≥40), residents of long-term care facilities.
  2. Those likely to transmit influenza to the at-risk groups above: healthcare workers, household contacts of at-risk people, out-of-home care givers of at-risk peopl.
  3. All pregnant women at any stage of pregnancy.
  4. People who have close, regular contact with pigs, poultry or water fowl.

Preventative benefits of flu vaccine

The preventative benefits of influenza vaccine are numerous. Influenza vaccination is currently the best available measure in preventing influenza infection. In addition, influenza vaccination can result in milder illness when infected with influenza.

In vulnerable groups such as the elderly, influenza vaccination can reduce the risk of influenza associated hospitalisation. A meta-analysis of influenza vaccine in community-dwelling adults ≥65 years of age found an effectiveness of 35% against ILI, 33% against pneumonia and influenza hospitalisations, 47% against pneumonia and influenza mortality, and 50% against all-cause mortality Additional studies have demonstrated that influenza vaccine reduces influenza-related hospitalisation by up to 61% and deaths by up to 80% in older people.

There is also substantial evidence to support influenza vaccination of people with chronic diseases such as heart disease, diabetes mellitus and chronic lung disease. In people with coronary artery disease, influenza vaccination was associated with lower rates of subsequent major cardiac events such as death and hospitalisation due to cardiac failure or stroke. Influenza vaccination has also been shown to reduce hospitalisations by 79% in patients with diabetes and by 52% in patients with chronic lung disease.

Pregnant women are more likely to become very ill from influenza due to changes in their heart and lung function which occur during pregnancy. They are more likely to need admission to hospital and even to a Critical Care Unit. Contracting influenza in pregnancy may also lead to premature birth, low birth weight and even death of a baby in utero. In addition, infants under 6 months of age have the highest rate of hospitalisation and death from influenza. Influenza vaccination helps protect women and their babies during and after pregnancy. Studies that looked at influenza vaccine effectiveness in pregnant women found that vaccination reduced the risk of influenza-associated acute respiratory tract infections in these women by about 50%. Maternal vaccination also protects the baby from influenza after birth as antibodies are passed from the mother to her developing baby during her pregnancy.

Influenza vaccine is recommended for healthcare workers (HCWs) to protect them from getting influenza and to reduce transmission of influenza from them to their family, colleagues and patients. HCWs are at increased risk of exposure and hence influenza infection compared to the general adult population, with approximately 20% of HCWs infected every year. HCWs care for elderly and at risk patients who respond less well to influenza vaccine. These vulnerable patients rely on the immunity of those who care for them to keep them safe. Studies have shown that during hospitalisation, patients are 5-35 times more likely to acquire influenza if exposed to infected patients or healthcare workers. The literature reports on many flu outbreaks within hospitals and long term care facilities where unvaccinated healthcare workers are likely to have infected patients and facilitated the spread of the disease. In contrast, institutions with high levels of healthcare worker immunisation have shown reduced rates of influenza-like illness, hospitalisation and deaths from flu in the elderly, and a reduction in healthcare worker sick leave. Increasing immunisation rates among healthcare workers and caregivers of the elderly and finding more effective vaccine for elderly people are likely to significantly improve disease prevention in this population.

Preliminary estimates suggest that the effectiveness of this year’s vaccine is in line with previous years

All HCWs should receive influenza vaccination, unless contraindicated. In addition, the HSE has set a target of 40% vaccination uptake among HCWs for the 2016/2017 influenza season. However, mid-season data on vaccine uptake for HCWs suggests sub-optimal results with vaccine uptake in hospital-based HCWs at 28.3% and vaccine uptake among HCWs in long term care facilities at 27.3%.

Influenza Vaccine Effectiveness

While vaccine effectiveness (VE) can vary, studies show that the vaccine reduces the risk of influenza illness by about 50% to 60% among the overall population during seasons when most circulating flu viruses are similar to the virus strains contained in the vaccine. Two factors play an important role in determining the likelihood that flu vaccine will protect a person from flu illness; the characteristics of the person being vaccinated (such as their age and health) and the similarity or “match” between the influenza strains in the flu vaccine that season and the influenza strains circulating in the community. Protection lasts for approximately one year. As the vaccine composition changes year on year, annual vaccination is required.

The main influenza subtype circulating in Ireland this season to date is influenza A(H3N2). Two genetic groups of influenza A(H3N2) are circulating in Europe; the pre-existing clade A/Hong Kong/4801/2014 3C.2a (contained in the vaccine) and a new emerging clade A/Bolzano/7/2016 3C.2a1. This new circulating clade is found to be antigenically similar to the vaccine strain and so should offer protection against the circulating influenza virus. A late co-circulation of A(H1N1)pdm09 virus during this season cannot be excluded, since low but increasing numbers of detections have already been reported by six countries in Europe. Influenza B virus may also circulate later in the season. Hence for unvaccinated persons in risk groups, including HCWs, it is still not too late to get the influenza vaccine which also provides protection against influenza A(H1N1)pdm09 and influenza B.

Even though the circulating strains of influenza A(H3N2) appear to be antigenically similar to the vaccine strain, vaccine effectiveness studies are undertaken to estimate the VE in the population and in population subgroups. Preliminary estimates suggest that the effectiveness of this year’s vaccine is in line with previous years. Between 2011 and 2015, VE estimates against A(H3N2) have ranged from 11% to 42%. This year early indications from Sweden and Finland suggest current vaccine effectiveness of 24–26% among the elderly. Effectiveness is always partial and the use of antivirals for the treatment of laboratory-confirmed or probable cases of influenza should be considered for vaccinated and non-vaccinated patients at risk However, the vaccine does offer protection against hospitalisation and deaths in the elderly as mentioned previously, hence the importance of this group receiving the vaccine. Their carers and HCWs should also be vaccinated in order to add an additional layer of protection.

The Health Protection Surveillance Centre (HPSC) provides extensive guidance on influenza including the following:
•    Intensive Care Guidance
•    Schools’ Guidance
•    Pregnancy Guidance
•    Clinical Management Algorithms
•    Residential Care Facilities Guidance
•    Antiviral Treatment and Prophylaxis Guidance

In summary, influenza circulates in the winter for up to three months and causes significant morbidity and mortality and an increased burden on the health care system. Outbreaks in long term care facilities can continue beyond this period late into the season. Vaccine is currently the best available mode of prevention and while it is not 100% protective against laboratory-confirmed influenza, it offers significant protection against severe morbidity and mortality, especially in the elderly and at risk groups. Protection increases with increased vaccine coverage in the population, hence the importance of vaccination of those in high risk groups including HCWs and carers of these groups. In addition, the use of antivirals for the treatment of laboratory-confirmed or probable cases of influenza should be considered for vaccinated and non-vaccinated patients at risk.

1. Influenza vaccine is available free of charge to those aged 65 and older and those in additional risk groups outlined above. However, an administration fee is charged by GPs and pharmacists to those who do not have a GMS card or GP doctor visit card (DVC).