Flu Shots Explained: What They Are, What Canada Recommends, and What the Science Says
Every fall the same questions come back. What is actually in a flu shot? Do I really need one every year? Does it work, and is it safe? This guide answers them from the official sources: the National Advisory Committee on Immunization (NACI), the Public Health Agency of Canada, Ontario's Ministry of Health and Peel Public Health. It also explains the types of flu, from influenza A and B to H1N1 and H3N2.
Quick answer
A flu shot teaches your immune system to recognize the flu viruses expected to circulate this season, so your body can fight them off faster if you're exposed. NACI recommends a flu vaccine every year for everyone 6 months of age and older who has no reason to avoid it, because the viruses change and protection fades over time. It takes about 2 weeks to reach its best protection. You cannot get the flu from the flu vaccine; side effects are usually mild and gone in a few days. In Ontario the shot is free for anyone 6 months and older who lives, works or goes to school in the province. Sources: canada.ca, ontario.ca.
Flu shots in stock in Mississauga. Flu shots are in stock at our walk-in clinic and our on-site pharmacy, Main Drug Mart, for kids, adults and seniors 65+. Children under 2 are vaccinated by a doctor at the walk-in clinic. Mon-Fri 9 AM-7 PM · Sat 10 AM-3 PM.
Looking for who is eligible in Ontario, where to go in Mississauga, and the three options for seniors? That is covered in our Flu Shot Season 2026 guide. Not sure whether it's a cold, the flu or COVID-19? See Cold, Flu or COVID-19? Symptoms and When to Come In. This post goes deeper into how the vaccine works and the evidence behind it.
What is a flu shot, and how does it work?
The flu (seasonal influenza) is a respiratory illness caused by influenza A and B viruses, with outbreaks in Canada every fall and winter. A flu vaccine shows your immune system the outer proteins of the flu viruses expected that season, so it can build antibodies before you meet the real thing.
The shot: inactivated vaccine
Most flu vaccines in Canada are inactivated vaccines given by needle in the arm. NACI describes them as made of disrupted virus particles: the virus is broken apart, and in some vaccines purified down to its surface proteins. The key ingredient is a standard amount of hemagglutinin (HA), a protein on the outside of the virus that your antibodies learn to recognize. NACI classes these shots as non-live vaccines.
The nasal spray: live attenuated vaccine
A nasal spray vaccine (live attenuated influenza vaccine) is authorized in Canada for ages 2 to 59. It uses weakened viruses that are cold-adapted, so they multiply in the lining of the nose rather than the lungs, and NACI says they do not produce flu-like illness. Ontario's free program for 2026-2027 lists inactivated shots only, so the nasal spray is not part of it this season.
Why you need a new one every year
Two reasons, both from the Canadian Immunization Guide. First, flu viruses change over time, so the vaccine is reformulated every year to match the strains the World Health Organization expects to circulate. Second, the immune response to the vaccine is temporary and may not last beyond a year.
How long it takes to work
Protective antibody levels are reached about 2 weeks after vaccination, though there may be some protection before then. Children 6 months to under 9 years getting a flu vaccine for the first time need 2 doses at least 4 weeks apart; everyone else needs 1 dose a season.
Sources: NACI, Summary: Statement on seasonal influenza vaccines for 2026-2027 (May 2026); NACI, Statement on seasonal influenza vaccines for 2026-2027 (full statement); Canadian Immunization Guide, Influenza vaccines (updated Aug 2026); Ontario Universal Influenza Immunization Program (updated Sep 2026).
Why get a flu shot?
Most people recover from the flu in 7 to 10 days, but it can also lead to pneumonia, heart and lung complications, hospital stays and death. The Public Health Agency of Canada's own figures:
Ontario's flu page uses a different estimate of about 12,200 hospitalizations a year (ontario.ca).
Older adults carry most of that burden. In the 2024-2025 season, adults 65 and older accounted for 59% of flu-related hospitalizations and 83% of flu-related deaths in Canada. Young children are also hit hard: children under 5 had the second-highest flu hospitalization rate that season. The flu is also linked to a higher risk of heart attack and stroke in the 15 days after infection, and NACI notes that the vaccine may help protect against heart attacks and strokes in high-risk groups such as people who already have heart disease.
It protects the people around you, too. If you're vaccinated, you're less likely to pass the flu on. That matters most for babies under 6 months, who are too young for the vaccine, and for older or chronically ill family members. Ontario notes that babies get some protection if their parent had a flu shot during pregnancy. The full list of higher-risk groups is in our Flu Shot Season 2026 guide.
Sources: Flu (seasonal influenza): Spread, prevention and risks; Flu: For health professionals; NACI statement 2026-2027 (full statement and summary); Canadian Immunization Guide; Flu: Get your flu vaccine; The flu (ontario.ca).
What Canada and Ontario recommend for 2026-2027
NACI (national)
NACI's statement for the 2026-2027 season, published in May 2026, keeps the core recommendation unchanged: the flu vaccine should be offered every year to anyone 6 months of age or older who does not have a contraindication. NACI says it matters most for three groups:
- People at high risk of complications or hospitalization: children 6 to 59 months, adults and children with certain chronic health conditions, pregnant people, residents of nursing homes and other chronic care facilities, adults 65 and older, and First Nations, Inuit and Métis communities.
- People who could pass the flu to someone at high risk: health care and other care providers, household contacts of high-risk people (including homes with a baby under 6 months or a newborn on the way), regular childcare providers for children under 5, and people serving closed settings such as ship crews.
- Others: people who provide essential community services, and people whose work or recreation exposes them to avian influenza A(H5N1).
On which vaccine to use, NACI says any age-appropriate flu vaccine should be used for people 6 months and older. The one preference is for adults 65 and older: a high-dose or adjuvanted vaccine should be offered when available, because these give better protection than the standard dose in that age group. (NACI also names a recombinant vaccine for this group, but the Canadian Immunization Guide confirms no recombinant flu vaccine is authorized in Canada any more.) If a preferred product isn't available, any vaccine authorized for that age should be used. For adults 18 to 59, NACI notes some evidence that the shot may work better than the nasal spray.
Ontario's Universal Influenza Immunization Program
Ontario offers the flu vaccine free each year to everyone 6 months and older who lives, works or goes to school in the province. For 2026-2027, high-risk and priority groups could start getting it the week of September 28, 2026, and everyone else from October 26, 2026. The publicly funded vaccines this season are a trivalent inactivated vaccine for 6 months and older, plus high-dose and adjuvanted trivalent vaccines for 65 and older. Ontario's message to seniors: any of the available flu vaccines is better than staying unvaccinated.
Peel Public Health
Peel Public Health confirms the general-population start date of October 26, 2026 (peelregion.ca).
Sources: NACI summary and full statement for 2026-2027 (May 2026); Canadian Immunization Guide, Influenza vaccines; Universal Influenza Immunization Program (ontario.ca, updated 25 Sep 2026); The flu (ontario.ca, updated 13 Oct 2026); Flu (peelregion.ca).
Ready to get yours? Flu shots are in stock at Medicare Clinic in Mississauga: at the walk-in clinic and at our on-site pharmacy, Main Drug Mart, for kids, adults and seniors 65+. Children under 2 are vaccinated by a doctor at the walk-in clinic.
What Canadians say: why people skip the flu shot
The Public Health Agency of Canada surveys adults every flu season. The latest results, for 2025-2026 (4,149 adults, interviewed in January and February), show:
| Group | Got a flu vaccine, 2025-2026 |
|---|---|
| All adults | 34% |
| Seniors 65+ | 64% |
| Adults 18-64 with a chronic medical condition | 36% |
| Adults 18-64 without a chronic medical condition | 21% |
Canada's goal is to vaccinate 80% of people at higher risk, such as seniors and adults with chronic conditions, and both groups still fall short of it. Most people who did get vaccinated went to a pharmacy (60%) or a doctor's office (14%). Among those who didn't, the main reasons were:
- Not feeling at risk: 28%
- Not getting around to it: 19%
- Safety concerns: 6%
The previous survey, for 2023-2024, also asked about beliefs. Most adults (87%) agreed the flu shot is safe, yet 43% believed they could get the flu from the flu vaccine, 40% thought it was better to build natural immunity by catching the flu, and 34% felt the vaccine does not protect them well enough. These are understandable concerns, and each one has a clear answer in the evidence:
"I'm healthy, so I'm not really at risk."
Healthy people are less likely to end up in hospital, but anyone can catch the flu and pass it on, including to a baby too young to be vaccinated or a grandparent at high risk. That is why NACI recommends the vaccine for everyone 6 months and older, not only for high-risk groups.
"I just never got around to it."
The most common practical barrier, and the easiest to fix. NACI's latest review found that getting vaccinated in early fall gives lasting protection for most of the season, and the vaccine can still be given until the season ends. In Ontario it is free, and pharmacies and clinics both give it.
"I'm worried about side effects."
Side effects are usually mild, like a sore arm for up to 2 days, and serious reactions are rare. The detail is in the science section below, including the official numbers on Guillain-Barré syndrome.
"The flu shot can give me the flu."
The Public Health Agency of Canada is direct: you cannot get the flu from the flu vaccine. The shot is a non-live vaccine, and the weakened viruses in the nasal spray do not produce flu-like illness.
"Natural immunity from catching the flu is better."
Catching the flu does produce immunity, but you take on the full risk of the illness to get it. The Public Health Agency of Canada's 2023-2024 flu vaccination survey report puts it this way: "vaccines are designed to provide protection with a controlled and safe level of exposure, minimizing the risk of severe symptoms or complications." Even the rare nerve condition people worry about after vaccination, Guillain-Barré syndrome, is more strongly linked to flu illness than to the vaccine.
"It doesn't work well enough to bother."
Fair question: the flu vaccine is not 100% effective, and how well it works changes every season. But even in a season with a poor match, vaccinated people are still more likely to be protected than unvaccinated people. The numbers are below.
Sources: Seasonal Respiratory Vaccination Coverage Survey, 2025-2026 report (updated 26 May 2026); Seasonal Influenza Vaccination Coverage in Canada, 2023-2024; Flu: Get your flu vaccine (canada.ca); NACI summary and full statement 2026-2027; Canadian Immunization Guide.
What the science says: effectiveness and safety
How effective is the flu shot?
Flu vaccine effectiveness varies from season to season. It depends on how well the vaccine strains match the viruses that circulate, which type and subtype of flu is going around, and the age and health of the person vaccinated. Canada measures it through the Canadian Sentinel Practitioner Surveillance Network, using data from British Columbia, Alberta, Ontario and Quebec.
The most recent full-season Canadian estimate published by NACI on canada.ca is for 2023-2024. That season, the vaccine reduced the risk of a medical visit for lab-confirmed flu by:
Effectiveness for the 2026-2027 season won't be known until the season is under way. Three more findings from the Canadian Immunization Guide: vaccinated people are still more likely to be protected than unvaccinated people even when the match is poor; for adults 65 and older, high-dose and adjuvanted vaccines appear more effective than the standard dose; and getting vaccinated in back-to-back years does not make the vaccine work worse (or better) than getting it in the current season only.
Is the flu shot safe? Common side effects
- Injection site: soreness that lasts up to 2 days is common. Adjuvanted vaccines tend to cause more injection-site reactions, which also clear within a few days.
- Whole body: muscle aches, headache, tiredness or feeling unwell, usually mild and short-lived. The high-dose vaccine causes these more often than the standard dose, but they are still mostly mild and short-lived.
- Nasal spray: a stuffy or runny nose is the most common reaction.
Serious reactions are rare. Ontario describes the flu shot as safe, including for children and during pregnancy and breastfeeding, and NACI says it has a good safety record in pregnancy. Peel Public Health lists the warning signs of a severe reaction, which is very rare: trouble breathing, swelling of the face or mouth, hives, seizures, or a fever over 40°C. Call 911 if you're having a medical emergency.
Guillain-Barré syndrome (GBS)
The Canadian Immunization Guide puts the risk of GBS after a flu vaccine at about 1 extra case per million vaccinations. The risk from flu illness itself is higher: about 17 cases per million flu-related health care visits. People who developed GBS within 6 weeks of a previous flu vaccine, with no other cause found, should generally avoid future flu vaccines and talk to their doctor.
Egg allergy
Most flu vaccines are made using eggs, but an egg allergy is not a reason to skip the flu vaccine. NACI's guidance is that people with an egg allergy can get any flu vaccine, including egg-based ones and the nasal spray, at the full dose, with no skin test first, even if they've had a severe reaction to egg in the past, and in any vaccination setting. (A true allergy to the vaccine itself, or a past anaphylaxis to a flu vaccine, is different: talk to your doctor.)
Common myths, answered
| Myth | What the official sources say |
|---|---|
| "The flu shot gives you the flu." | You cannot get the flu from the flu vaccine (canada.ca). |
| "I had one last year, so I'm covered." | Protection decreases over time and the strains can change, so a new vaccine is needed every year (NACI). |
| "Getting it every year weakens it." | Vaccination in consecutive seasons has no negative or positive effect on effectiveness compared with the current season only (Canadian Immunization Guide). |
| "I can't have it with my COVID-19 shot." | Flu vaccines can be given the same day as, or any time before or after, other vaccines, including COVID-19 vaccines (NACI). |
| "It's too late once flu season starts." | Getting it before the season is strongly preferred, but it can still be given until the end of the season (Canadian Immunization Guide). |
| "The flu shot also protects against bird flu." | Seasonal flu vaccines do not protect against avian influenza A(H5N1) (canada.ca, NACI). |
Sources: Canadian Immunization Guide, Influenza vaccines (updated Aug 2026); NACI statement 2025-2026, section on vaccine effectiveness, for the 2023-2024 Sentinel Practitioner Surveillance Network estimates; NACI full statement and summary 2026-2027; Flu: Get your flu vaccine (canada.ca); The flu (ontario.ca); Flu (peelregion.ca).
Types of flu: influenza A, B, C and D
There are four types of influenza virus. Only A and B cause the seasonal epidemics we call flu season.
| Type | What to know |
|---|---|
| Influenza A | Split into subtypes by its two surface proteins, hemagglutinin (H) and neuraminidase (N). The two A subtypes circulating in people now are A(H1N1) and A(H3N2). The current H1N1 is often written A(H1N1)pdm09 because it caused the 2009 pandemic. Only influenza A viruses are known to have caused pandemics. Wild birds are the main reservoir for most influenza A viruses. |
| Influenza B | No subtypes; instead two lineages, B/Victoria and B/Yamagata. There have been no confirmed natural cases of B/Yamagata anywhere in the world since March 2020. In Canada, influenza B shows up most in children and teens: in 2024-2025, people under 19 accounted for 50% of influenza B detections. |
| Influenza C | Detected less often and usually causes mild infections, so it is not considered a public health concern (WHO). |
| Influenza D | Mainly affects cattle and is not known to infect or cause illness in people (WHO). |
Which one went around last season? Canada's 2024-2025 flu season started later than usual, in mid-December 2024, and peaked in late February 2025. Influenza A made up 88% of detections, with A(H1N1)pdm09 the main strain for the second season in a row. Influenza B (Victoria) came later, at much lower levels.
Is influenza A or B worse? It depends on who gets sick and which strain is circulating. NACI notes that older adults face a higher burden from influenza A than other age groups. Children and teens show up far more in influenza B: in 2024-2025, under-19s made up 12% of all flu hospitalizations in Canada but 43% of influenza B hospitalizations.
Why Canada's flu vaccines now protect against 3 strains, not 4
For years, many flu vaccines were "quadrivalent", covering four strains: one H1N1, one H3N2 and one from each B lineage. Because B/Yamagata has not been detected since March 2020, the World Health Organization recommended dropping it from all flu vaccines, and NACI supported the move to "trivalent" (three-strain) vaccines. The flu vaccines NACI lists as authorized and available in Canada for 2026-2027 are all trivalent: one A(H1N1), one A(H3N2) and one B/Victoria strain. The B/Yamagata strain has been removed from flu vaccines available in Canada.
Flu vaccine types available in Canada
| Vaccine type | Who it's for | What's different | On Ontario's free 2026-2027 list? |
|---|---|---|---|
| Standard-dose inactivated (egg-based) | 6 months and older | The regular flu shot: 15 µg of HA per strain. | Yes (trivalent inactivated vaccine, 6 months+) |
| Cell-culture-based inactivated | 6 months and older | Viruses grown in mammalian cells instead of eggs; same standard dose. | Not named separately on Ontario's list |
| High-dose inactivated | 65 and older | 60 µg of HA per strain, four times the standard amount, for a stronger immune response. | Yes, 65+ |
| Adjuvanted inactivated | 65 and older | Standard antigen plus an adjuvant (MF59) that boosts the immune response. A children's version is authorized but has not been available in Canada since 2019. | Yes, 65+ |
| Live attenuated nasal spray | 2 to 59 years | Weakened live viruses sprayed into the nose; not for pregnancy, severe asthma or weakened immune systems. | No |
| Recombinant | n/a | No longer authorized or available in Canada. | No |
Not every authorized vaccine is sold in every province each year. Your clinic or pharmacist will tell you which one suits your age and health.
Sources: World Health Organization, Influenza (seasonal) fact sheet; Canadian Immunization Guide, Influenza vaccines; NACI full statement 2026-2027 (epidemiology, vaccine products); NACI statement 2025-2026 (transition to trivalent vaccines); Universal Influenza Immunization Program (ontario.ca).
Frequently asked questions
What exactly does a flu shot do?
It shows your immune system the surface proteins of this season's expected flu viruses so it can make antibodies ahead of time. That lowers your chance of getting the flu and helps prevent serious complications if you do. Protection peaks about 2 weeks after the shot. Sources: canada.ca, Canadian Immunization Guide.
Is a flu shot necessary every year?
Yes. NACI recommends a flu vaccine every year for everyone 6 months and older without a contraindication, because protection fades over time and the circulating strains change. The World Health Organization reviews the strains each year and they are often updated. Sources: NACI, Canadian Immunization Guide.
How effective is the flu shot?
It varies by season. In 2023-2024, the most recent full-season Canadian estimate published by NACI, the vaccine cut the risk of a medical visit for lab-confirmed flu by 46%. Effectiveness for 2026-2027 won't be known until the season is under way. Even in a poorly matched year, vaccinated people are more likely to be protected than unvaccinated people. Sources: NACI statement 2025-2026, Canadian Immunization Guide.
What are the downsides of getting a flu shot?
The common ones are a sore arm for up to 2 days and, sometimes, mild muscle aches, headache or tiredness that pass quickly. Serious reactions are rare. The risk of Guillain-Barré syndrome is about 1 extra case per million vaccinations, lower than the risk from flu illness itself. Source: Canadian Immunization Guide.
Can the flu shot make you sick?
It cannot give you the flu. The shot is a non-live vaccine, and the weakened viruses in the nasal spray do not cause flu-like illness. Side effects are usually mild and go away on their own within a few days. Sources: canada.ca, NACI.
What are the different types of flu vaccines available?
In Canada: standard-dose and cell-culture-based shots for 6 months and older, high-dose and adjuvanted shots for adults 65 and older, and a live nasal spray for ages 2 to 59. The vaccines listed as available for 2026-2027 are trivalent (three strains). Ontario's free 2026-2027 program covers the standard-dose shot for 6 months and older and the high-dose and adjuvanted shots for 65 and older. Sources: Canadian Immunization Guide, ontario.ca.
Is H1N1 influenza A or B?
H1N1 is a subtype of influenza A. The two influenza A subtypes circulating in people now are A(H1N1) and A(H3N2); influenza B is divided into lineages (Victoria and Yamagata) instead of subtypes. Sources: WHO, Canadian Immunization Guide.
How long will my flu shot last?
About one season. The immune response is temporary and may not last beyond a year, which is why a new shot is recommended every fall. NACI's latest review found that getting vaccinated in early fall gives lasting protection for most of the season. Sources: Canadian Immunization Guide, NACI 2026-2027.
Ready when you are. Flu shots are in stock at Medicare Clinic's walk-in clinic and Main Drug Mart pharmacy in Mississauga, for kids, adults and seniors 65+.
Sources: NACI, Statement on seasonal influenza vaccines for 2026-2027 and its summary (Public Health Agency of Canada, May 2026); NACI, Statement on seasonal influenza vaccines for 2025-2026; Canadian Immunization Guide, Part 4, Influenza vaccines (updated 26 Aug 2026); Flu (seasonal influenza): Get your flu vaccine; Spread, prevention and risks; For health professionals (canada.ca); Seasonal Respiratory Vaccination Coverage Survey 2025-2026; Seasonal Influenza Vaccination Coverage in Canada, 2023-2024; The flu and Universal Influenza Immunization Program (ontario.ca); Flu (peelregion.ca); Influenza (seasonal) fact sheet (who.int).
This guide is general information, not medical advice. Talk to your doctor, nurse practitioner or pharmacist about your own health.