Influenza vaccine: Canadian Immunization Guide
Key information

# What
- Influenza is a respiratory infection caused primarily by influenza A and B viruses.
- Seasonal influenza epidemics occur annually in Canada, generally in the late fall and winter months.
- Prior to the COVID-19 pandemic, influenza occurred globally with an annual attack rate estimated at 5% to 10% in adults and 20% to 30% in children.
- Common symptoms of influenza include fever, cough and myalgia. Most people will recover within a week to 10 days, but some people are at greater risk of complications, such as pneumonia.
- There are 10 influenza vaccines authorized and available for use in Canada: 8 inactivated influenza vaccines (IIV), a recombinant influenza vaccine (RIV), and a live attenuated influenza vaccine (LAIV). Some protect against 3 strains of influenza (i.e., trivalent) and others protect against 4 strains of influenza (i.e., quadrivalent).
- The influenza vaccines are safe and well-tolerated.

# Who
- Influenza vaccination should be offered annually to anyone 6 months of age and older who does not have contraindications to the vaccine.
- Influenza vaccination is particularly recommended for:
- people at high risk of severe disease, influenza-related complications, or hospitalization
- people capable of transmitting influenza to those at high risk
- people who provide essential community services
- people in direct contact with poultry infected with avian influenza during culling operations

# How
- Adults and children 9 years of age and older should receive 1 dose of influenza vaccine each year.
- Children 6 months to less than 9 years of age who have never received influenza vaccine in a previous influenza season should be given 2 doses of influenza vaccine in the current season.
- Children 6 months to less than 9 years of age who have been vaccinated with at least 1 dose of seasonal influenza vaccine in any previous season should receive 1 dose of influenza vaccine per season thereafter.

# Why
- Vaccination is the most effective way to prevent influenza and its complications that can lead to severe disease, hospitalization and death.
- Vaccination helps prevent the spread of influenza.
- Annual vaccination is required because influenza viruses are constantly evolving and the body's immune response to influenza vaccination may not persist beyond a year. A new vaccine is designed each year to target the changes in the circulating influenza viruses.
Epidemiology

# Disease description

## Infectious agent
There are 2 main types of influenza virus that cause seasonal epidemics in humans: A and B. Influenza A viruses are classified into subtypes based on 2 surface proteins: hemagglutinin (HA) and neuraminidase (NA). Three subtypes of HA (H1, H2, and H3) and 2 subtypes of NA (N1 and N2) are recognized among influenza A viruses as having caused widespread human disease.
There are 2 antigenically distinct lineages of influenza B viruses, B/Yamagata/16/88-like and B/Victoria/2/87-like viruses. Viruses from both lineages contribute variably to influenza illness each year.
Over time, antigenic variation (drift) of strains occurs within an influenza A subtype or a B lineage. Consequently, seasonal influenza vaccines need to be reformulated annually, to better match the circulating vaccine strains.

## Reservoir
The primary reservoir of most influenza A viruses is wild birds. In humans, currently only 3 type/subtype viruses , are circulating; for these, humans are the reservoir.

## Transmission
Influenza is primarily transmitted by aerosols and droplets spread through coughing or sneezing, and through direct or indirect contact with respiratory secretions. The incubation period is usually about 2 days but can range from 1 to 4 days. Adults may be able to spread influenza to others from 1 day before symptom onset to approximately 5 days after symptoms start. Children and people with weakened immune systems may be infectious longer.

## Risk factors
The people at greatest risk of influenza-related complications are adults and children with chronic health conditions, residents in long term care facilities, adults 65 years of age and older, children 0 to 59 months of age, individuals who are pregnant, and Indigenous Peoples.

## Seasonal and temporal patterns
Seasonal influenza activity in Canada begins to increase over the fall, and peaks in the winter months. The influenza season can last for many months, and more than 1 influenza strain typically circulates each season. Sporadic cases and occasional outbreaks may occur outside the typical influenza season.

## Spectrum of clinical illness
Influenza infection may be asymptomatic or present as mild disease; conversely, it can manifest as severe disease and result in death. Symptoms typically include the sudden onset of fever, cough, and myalgia. Other common symptoms include headache, chills, loss of appetite, fatigue, and sore throat. Nausea, vomiting, and diarrhea may also occur, especially in children. Most people will recover within a week to 10 days. In some, influenza infection may lead to complications including pneumonia, respiratory failure, cardiovascular complications, or worsening of underlying chronic medical conditions. Influenza infection is also associated with an increased risk of myocardial infarction, stroke and Guillain-Barre syndrome (GBS).

## Disease distribution
Worldwide, annual epidemics result in approximately 1 billion cases of influenza, 3 to 5 million cases of severe illness, and 290,000 to 650,000 deaths. Historically, the global annual attack rate was estimated to be 5% to 10% in adults and 20% to 30% in children.
Together, influenza and pneumonia are among the top 10 leading causes of death in Canada. Prior to the COVID-19 pandemic, an average of 50,000 laboratory-confirmed cases of influenza were reported each year. This is an underestimate of the actual number of cases as most influenza infections go unreported. Although the burden of influenza-associated illness and death can vary from year to year, it is estimated that there are an average of 12,200 hospitalizations related to influenza and approximately 3,500 deaths attributable to influenza annually in Canada. Information on current influenza activity in Canada can be found on the 
Preparations authorized for use in Canada

# Inactivated influenza vaccine (IIV)

## Standard-dose, egg-based, quadrivalent inactivated influenza vaccine (IIV4-SD)
- Afluria Tetra, Seqirus Ltd. (IIV4)
- Flulaval Tetra, GlaxoSmithKline Inc. (IIV4)
- Fluzone Quadrivalent, Sanofi Pasteur Inc. (IIV4)
- Influvac Tetra, BGP Pharma ULC. (IIV4)

## Standard-dose mammalian cell culture-based quadrivalent inactivated influenza vaccine (IIV-cc)
- Flucelvax Quad, Seqirus Ltd. (IIV4-cc)

## Adjuvanted inactivated trivalent influenza vaccine (IIV-Adj)
- Fluad Pediatric, (Pediatric dose ), Seqirus Ltd. (IIV3-Adj)
- Fluad, Seqirus Ltd. (IIV3-Adj)

## High-dose inactivated influenza vaccine (IIV-HD)
- Fluzone, (High-Dose ), Sanofi Pasteur Ltd. (IIV4-HD)

## Recombinant influenza vaccine (RIV)
- SupemtekTM, Sanofi Pasteur Ltd. (RIV4)

## Live attenuated influenza vaccine (LAIV)
- FluMist Quadrivalent, AstraZeneca Canada. (LAIV4)
Trivalent vaccines contain 1 A(H1N1) strain, 1 A(H3N2) strain, and 1 influenza B strain from 1 of 2 lineages, Victoria or Yamagata. Quadrivalent vaccines contain the strains in the trivalent vaccine plus an influenza B strain from the other lineage.
The antigenic characteristics of circulating influenza virus strains provide the basis for the . Manufacturers that distribute influenza vaccines in Canada ensure the vaccines for the upcoming influenza season contain the WHO's recommended antigenic strains for the Northern Hemisphere. Vaccine producers may also use antigenically equivalent strains. Not all influenza vaccines authorized for use in Canada are available in Canada.
A summary of the characteristics of influenza vaccines available in Canada during the 20232024 influenza season can be found in  of the NACI Statement on Seasonal Influenza Vaccine. For complete prescribing information, consult the product monograph or information contained within Health Canada's authorized product monographs available through the . Refer to Table 1 in  in Part 1 for a list of all vaccines authorized for use in Canada and their contents.
Immunogenicity, efficacy and effectiveness

# Immunogenicity
Antibody response after vaccination depends on several factors, including the age of the recipient, prior and subsequent exposure to antigens, and the presence of immune compromising conditions. Vaccine-induced immune response is not as robust in young children, older adults and persons with immune compromising conditions as it is in other people. Protective levels of humoral antibodies, which correlate with protection against influenza infection, are achieved approximately 2 weeks after vaccination; however, there may be some protection afforded before that time.

# Efficacy and effectiveness
Influenza vaccine has been shown to be efficacious in providing protection against influenza infection and illness; however, the effectiveness of the vaccine can vary from season to season and by influenza vaccine strain type and subtype. Influenza vaccine effectiveness depends on how well the vaccine strains match with circulating influenza viruses, the type and subtype of the circulating virus, as well as the health and age of the individual receiving the vaccine. Nevertheless, even when there is a less-than-ideal match or lower effectiveness against 1 strain, vaccine recipients, particularly people at high risk of influenza-related complications and hospitalization, are still more likely to be protected compared to those who are unvaccinated.
In general, influenza vaccination in consecutive seasons does not have a negative or positive effect on vaccine effectiveness in comparison to vaccination in the current season only.
Recommendations for use

# Routine schedule
Seasonal influenza vaccine should be offered annually to everyone 6 months of age and older who does not have contraindications to the vaccine, irrespective of vaccination in previous seasons.  provides age group-specific recommendations. Any of the age-appropriate influenza vaccine types available for use may be considered for people without contraindications to the vaccine.
Influenza vaccines do not confer sufficient protection to make the vaccine useful in infants less than 6 months of age. Children 6 months to less than 9 years of age who have not previously received at least 1 dose of the seasonal influenza vaccine require 2 doses of influenza vaccine, with a minimum of 4 weeks between doses. Only 1 dose of influenza vaccine per season is recommended for everyone else.
Vaccination before the onset of the influenza season is strongly preferred, as delayed administration may result in lost opportunities to prevent infection from exposures that occur prior to vaccination. However, influenza vaccine may still be administered until the end of the season.
- IIV4-SD
- IIV4-cc
- IIV4-cc
- LAIV4
 - LAIV4 may be given to children with:
	+ stable, non-severe asthma
	+ cystic fibrosis who are not being treated with immunosuppressive drugs (e.g., prolonged systemic corticosteroids)
	+ stable HIV infection, i.e., if the child is currently being treated with ART (i.e., HAART) for at least 4 months and has adequate immune function
- LAIV should not be used in children or adolescents for whom it is contraindicated or for whom there are warnings and precautions such as those with:
	+ severe asthma (defined as currently on oral or high-dose inhaled glucocorticosteroids or active wheezing)
	+ medically attended wheezing in the 7 days prior to vaccination
	+ current receipt of aspirin or aspirin-containing therapy
	+ immune compromising conditions, with the exception of stable HIV infection, i.e., if the child is currently being treated with HAART for at least 4 months and has adequate immune function
	+ pregnancy
		- in pregnancy, IIV should be used instead
- IIV4-cc
- RIV4
- LAIV4
- IIV4-cc
- RIV4
- IIV4-SD
- IIV4-HD
- IIV4-cc
- RIV4
Vaccination of specific populations
To reduce the morbidity and mortality associated with influenza, immunization programs may focus on people at high risk of influenza-related complications (refer to ).
People at high risk of influenza-related complications or hospitalization* All children 6 to 59 months of age
- Adults and children with the following chronic health conditions:
	+ Cardiac or pulmonary disorders (includes bronchopulmonary dysplasia, cystic fibrosis, and asthma)
	+ Diabetes mellitus and other metabolic diseases
	+ Cancer, immune compromising conditions (due to underlying disease, therapy, or both, such as solid organ transplant or hematopoietic stem cell transplant recipients)
	+ Renal disease
	+ Anemia or hemoglobinopathy
	+ Neurologic or neurodevelopmental conditions (includes neuromuscular, neurovascular, neurodegenerative, and seizure disorders , but excludes migraines and psychiatric conditions without neurological conditions)
	+ Morbid obesity (defined as BMI of 40 kg/m2 and over)
	+ Children 6 months to 18 years of age undergoing treatment for long periods with acetylsalicylic acid, because of the potential increase of Reye's syndrome associated with influenza
- All individuals who are pregnant
- People of any age who are residents of nursing homes and other chronic care facilities
- Adults 65 years of age and older
- Indigenous Peoples
- Household contacts, both adults and children, of individuals at high risk, whether or not the individual at high risk has been vaccinated:
- household contacts of individuals at high risk
- household contacts of infants less than 6 months of age, as these infants are at high risk but cannot receive influenza vaccine
- members of a household expecting a newborn during the influenza season
- Those providing regular child care to children 0 to 59 months of age, whether in or out of the home
- Those who provide services within closed or relatively closed settings to people at high risk (e.g., crew on a cruise ship)
Others* People who provide essential community services
- People who are in direct contact with poultry infected with avian influenza during culling operations

# Children 6 months to 59 months of age
Young children have a high burden of influenza-associated illness. The risk of serious infection and hospitalization is highest among the very young.

# Pregnant individuals
Pregnant individuals, at any stage of pregnancy, are recommended to receive any age-appropriate non-live influenza vaccine due to the increased risk of influenza associated morbidity in pregnancy and evidence of adverse neonatal outcomes associated with influenza infection during pregnancy. Moreover, vaccination of individuals who are pregnant protects their newborns from influenza and influenza-related hospitalization. It has also been shown to be protective against infants being born premature, small for gestational age, and low birth weight. To protect infants less than 6 months of age who are at high risk of influenza-related illness, the influenza vaccine should not only be offered to individuals who are pregnant, but to any household contacts and care providers of young infants.

# Older adults
Adults 65 years of age and older are at greater risk of more severe complications from influenza, and influenza-attributed mortality rates increase with age. High dose (IIV-HD) formulation is recommended for older adults as it provides better protection compared with the standard dose influenza vaccine in adults 65 years of age and older.

# Residents in nursing homes or other chronic care facilities
Residents of nursing homes and other chronic care facilities often have 1 or more chronic health conditions and live in institutional environments that may facilitate the spread of influenza.

# Persons with chronic health conditions
Certain chronic health conditions are associated with increased risk of influenza-related complications and hospitalization. Refer to . Influenza infection in individuals with certain chronic diseases can also lead to an exacerbation of the chronic condition.

# Immunocompromised persons
People who are immunocompromised have an increased risk of morbidity and mortality from influenza. All people 6 months of age and older who are immunocompromised are particularly recommended to receive an influenza vaccine every year. Although influenza vaccination can induce protective antibody levels in a substantial proportion of adults and children with immune compromising conditions, vaccine effectiveness may be lower than in healthy individuals.

# Indigenous Peoples
Influenza vaccination is particularly recommended for Indigenous Peoples who tend to have higher rates of influenza-associated hospitalization and death. The increased risk of severe outcomes may be related to the presence of chronic health conditions and/or delays in accessing healthcare. Susceptibility to infection may also be increased due to living conditions that favour transmission.

# Health care workers, care providers and other workers
Influenza vaccination is recommended for health care workers (HCWs) and other care providers include regular visitors, emergency response workers, those who work in continuing care or long-term care facilities or residences, those who provide home care for people at high risk, and students of related health care services. HCWs and other care providers who are potentially capable of transmitting influenza to those at high risk should receive annual vaccination with any age appropriate non-live influenza vaccine, regardless of whether the high-risk individual has been vaccinated. Vaccination of HCWs and other care providers decreases the vaccinated person's risk of illness, as well as the risk of transmission of influenza to patients at high risk of influenza-associated complications. Vaccination of HCWs and residents of nursing homes is associated with decreased risk of influenza outbreaks. Annual influenza vaccination is considered an essential component of the standard of care for all HCWs.
To minimize the disruption of services during annual influenza epidemics, all people who provide essential community services should consider annual influenza vaccination, as it can decrease work absenteeism due to respiratory and related illnesses.
Seasonal influenza vaccination will not prevent avian influenza but it is recommended for those working in direct contact with poultry infected with avian influenza during culling operations, as these individuals may be at increased risk of exposure to avian influenza infection. Preventing infection with human influenza strains may reduce the theoretical potential for human-avian reassortment of genes, should such workers become co-infected with both human and avian influenza viruses.

# Travellers
Influenza occurs year-round in the tropics. In temperate northern and southern countries, influenza activity generally peaks during the winter season (November to March in the Northern Hemisphere and April to October in the Southern Hemisphere). Influenza vaccine should be offered to travellers.
Vaccines prepared specifically for use in the Southern Hemisphere are not available in Canada, and the extent to which recommended vaccine components for the Southern Hemisphere may overlap with those in available Canadian formulations will vary. A decision for or against revaccination of travellers to the Southern Hemisphere between April and October, if they had already been vaccinated in the preceding fall or winter with the Northern Hemisphere's vaccine, depends on individual risk assessment, the similarity between the Northern and Southern Hemisphere vaccines, the similarity between the Northern Hemisphere vaccine strains and currently circulating strains in the Southern Hemisphere, and the availability of a reliable and safe vaccine at the traveller's destination.
Serologic testing
Serologic testing is not necessary or recommended before or after receiving seasonal influenza vaccine.
Administration practices

# Dose and route of administration
The dose and route for influenza vaccines vary and are detailed in the product monographs.
- With the exception of IIV4-HD, most unadjuvanted IIVs are administered as a 0.5 mL intramuscular (IM) injection for everyone 6 months of age and older.
- The following IIVs are administered as a 0.5 mL IM injection but are only authorized or recommended for certain age groups: Afluria Tetra (5 years and older), Influvac Tetra (3 years and older), and Flucelvax Quad (2 years and older).
- IIV4-HD (Fluzone High-Dose Quadrivalent) is administered as a 0.7 mL IM injection for adults 65 years of age and older.
- Adjuvanted IIV3 (Fluad) is administered as a 0.5 mL IM injection for adults 65 years of age and older. A pediatric formulation is also available (Fluad Pediatric) and is administered as a 0.25 mL IM injection for children 623 months of age.
- RIV4 (SupemtekTM) is administered as a 0.5 mL IM injection for adults 18 years of age and older.
- LAIV (FluMist Quadrivalent) is administered as 0.2 mL given intranasal (0.1 mL in each nostril) for individuals 2 to 59 years of age.

# Concurrent administration with other vaccines
All influenza vaccines, including LAIV, may be given at the same time as, or at any time before or after administration of other live or inactivated vaccines, including COVID-19 vaccines for those aged 6 months and older.

# Interchangeability of vaccines
If a child aged less than 9 years requires 2 doses of influenza vaccine in the same influenza season, it is preferable to use the same type of vaccine for both doses. However, if the type of vaccine used for the first dose is not available for the second dose, a different type of influenza vaccine may be provided. For more information refer to  in Part 1.

# Pre- and post-vaccination counselling
Storage requirements
Influenza vaccines should be stored at +2C to +8C and should not be frozen. For additional information, consult the product monographs available through Health Canada's . Refer to  in Part 1 for additional information.
Safety and adverse events

# Common and very common adverse events
Common adverse events occur in 1% to less than 10% of vaccinees. Very common adverse events occur in 10% or more of vaccinees.
With IM-administered influenza vaccines, mild and transient injection site reactions e.g., soreness at the injection site lasting up to 2 days, are common. Any systemic reactions e.g., myalgia, headache, fatigue and malaise, are usually mild and short-lived. Influenza vaccines with adjuvant tend to produce more extensive injection site reactions than unadjuvanted, but these reactions are also generally mild and resolve within a few days. IIV-HD tends to induce higher rates of systemic reactions compared to IIV-SD, but most of these reactions are also mild and short-lived. The most common adverse reactions experienced by recipients of LAIV are nasal congestion and runny nose.

# Uncommon, rare and very rare adverse events
Uncommon adverse events occur in 0.1% to less than 1% of vaccinees. Rare and very rare adverse events occur, respectively, in 0.01% to less than 0.1% and less than 0.01% of vaccinees.
Serious adverse events are rare following influenza vaccination, and in most cases, data are insufficient to determine a causal association. Allergic responses to influenza vaccine are a rare consequence of hypersensitivity to some components of the vaccine or its container.

# Other reported adverse events and conditions

## Guillain-Barre syndrome
Studies suggest that the absolute risk of GBS in the period following seasonal and A(H1N1) pdm09 influenza vaccination is about 1 excess case per million vaccinations, and that the risk of GBS associated with influenza illness (about 17 cases per million influenza-coded health care encounters, which are a proxy for influenza illness) is higher than that associated with influenza vaccination.
Although the evidence considering influenza vaccination and GBS is inadequate to accept or reject a causal relation between GBS in adults and seasonal influenza vaccination, avoiding subsequent influenza vaccination of individuals known to have had GBS without other known etiology within 6 weeks of a previous influenza vaccination appears prudent at this time. However, the potential risk of GBS recurrence associated with influenza vaccination must be balanced against the risk of GBS associated with influenza infection itself and the benefits of influenza vaccination.

## Oculorespiratory syndrome
Oculorespiratory syndrome (ORS) consists of bilateral red eyes and 1 or more associated respiratory symptoms (cough, wheeze, chest tightness, difficulty breathing, difficulty swallowing, hoarseness, or sore throat) that starts within 24 hours of vaccination, with or without facial oedema. ORS was first identified during the 2000  2001 influenza season. Since then, there have been far fewer cases per year. ORS is not considered to be an allergic response. People who have an occurrence or recurrence of ORS upon vaccination do not necessarily experience further episodes with future vaccinations. Individuals who have experienced ORS without lower respiratory tract symptoms may be safely revaccinated with influenza vaccine. Individuals who have experienced ORS with lower respiratory tract symptoms should seek medical advice.

# Guidance on reporting adverse events following immunization
To ensure the ongoing safety of vaccines in Canada, reporting of adverse events following immunization (AEFIs) by vaccine providers and other clinicians is critical, and in some jurisdictions, reporting is mandatory under the law.
Vaccine providers are asked to report AEFIs through  officials and to check for specific AEFI reporting requirements in their province or territory. In general, any serious or unexpected adverse event felt to be temporally related to vaccination should be reported.
For influenza vaccines, the following AEFIs are of particular interest:
- ORS
- GBS within 6 weeks following vaccination

# Contraindications and precautions
Influenza vaccines are contraindicated in persons with a history of anaphylaxis after previous administration of the vaccine and in persons with proven immediate or anaphylactic hypersensitivity to any component, except egg, of the specific vaccine or its container. Refer to  in Part 1 for a list of all vaccines authorized for use in Canada and their contents.
Safety data confirm that egg-allergic individuals may be vaccinated against influenza using any influenza vaccine, including egg-based vaccines and LAIV, without prior influenza vaccine skin test and with the full dose, irrespective of a past severe reaction to egg and without any particular considerations, including vaccination setting.
Additional contraindications apply to the use of LAIV in people with certain health conditions:
- People with immune-compromising conditions due to underlying disease, therapy, or both, with the exception of children with stable HIV infection on anti-retroviral therapy (ART) and with adequate immune function
- People with severe asthma who are on oral or high dose inhaled glucocorticosteroids or have active wheezing or have medically attended wheezing in the 7 days of vaccination
- Children less than 24 months of age
- Children 2 to 17 years of age currently receiving aspirin or aspirin-containing therapy
- Pregnant individuals, because there is a lack of safety data at this time.
Precautions for the use of LAIV are as follows:
- LAIV should not be administered until 48 hours after antiviral agents active against influenza (e.g., oseltamivir, zanamivir) are stopped, and those antiviral agents, unless medically indicated, should not be administered until 2 weeks after receipt of LAIV so that the antiviral agents do not inactivate the replicating vaccine virus.
- If significant nasal congestion is present that might impede delivery of LAIV to the nasopharyngeal mucosa, IIV can be administered or LAIV can be deferred until resolution of the congestion.
- LAIV is not recommended for adults with the chronic health conditions identified in , IIV or RIV should be used instead.
- LAIV is not recommended for HCWs, IIV or RIV should be used instead.
- LAIV recipients should avoid close association with people with severe immune compromising conditions (e.g., bone marrow transplant recipients requiring isolation) for at least 2 weeks following vaccination, because of the theoretical risk for transmitting a vaccine virus and causing infection.
- LAIV recipients who are less than 18 years of age should avoid the use of aspirin-containing products for at least 4 weeks after receipt of LAIV.
As a precaution, influenza vaccination should be avoided in people who have developed GBS within 6 weeks of a previous influenza vaccination. Medical advice may be sought to balance the potential risk of GBS recurrence associated with influenza vaccination against the risk of GBS associated with influenza infection itself, and the benefits of influenza vaccination.
ORS is usually transient, resolving within 48 hours of onset. The only associated precaution is when lower respiratory symptoms accompany ORS, in which case expert review is required prior to subsequent immunization.
Influenza vaccination should not be delayed because of minor or moderate acute illness, with or without fever; however, in people with serious acute illnesses it may be postponed until their symptoms have abated.
Other considerations

# Drug interactions
Although influenza vaccine can inhibit the clearance of warfarin and theophylline, clinical studies have not shown any adverse effects attributable to these drugs in people receiving influenza vaccine. Statins have effects on the immune system in addition to their therapeutic cholesterol-lowering actions. Two published studies have found that adults who are regular statin users had an apparent decreased response to influenza vaccination as measured by reduced geometric mean titres (GMT) or reduced vaccine effectiveness against medically attended acute respiratory illness. Statins are widely used in the same adult populations who are also at-risk for influenza-related complications and hospitalizations. Therefore, if these preliminary findings are confirmed in future studies, concurrent statin use in adult populations could have implications for influenza vaccine effectiveness and how this use is assessed in the measurement of vaccine effectiveness.
Chapter creation process
This new chapter was developed based on the Statement on Seasonal Influenza Vaccine for 20232024 from the National Advisory Committee on Immunization (NACI). It incorporates key information from other recent NACI statements on influenza vaccines.