Why do some people get the flu after their shot? Do COVID boosters cause more side-effects?

Here are the essential flu vaccine questions answered

Every year, Canadians brace for flu season — and every year, the same questions come up. From cost to side-effects to timing, here are the key flu vaccine questions answered with up-to-date information for Canada, with a special focus on Ontario.

Do I have to pay for the flu vaccine in Canada? (Ontario specifics)

In Canada, and especially in Ontario, the seasonal flu vaccine is free for everyone six months of age and older who lives, works, or goes to school in the province with or without health card.

You can get it at:

• Pharmacies (ages 2+ in Ontario)

• Family doctors and walk-in clinics

• Public health units

If you have private or workplace insurance, it may also cover enhanced vaccines for adults who prefer them.

Are there any groups who should not get the flu vaccine this year?

The only reason not to get the flu vaccine is if you have had a life-threatening allergy to a previous flu vaccine or to one of its ingredients.

If you have an egg allergy, tell the person giving you the vaccine so they can ensure you receive a product that is safe for you.

How long does it take for the flu vaccine to take effect?

It takes roughly two weeks for the flu vaccine to reach peak effectiveness. Your immune system needs this time to build protective antibodies against the influenza viruses.

If I get the flu a few days after the vaccine, does it mean I already had it?

Not necessarily, but it is possible.

After you catch the flu virus, it usually hides in your body for a few days before you notice symptoms.

This means you may:

1. Be infected with the flu,

2. Get vaccinated while the virus is already incubating,

3. And then develop symptoms a few days later.

This is unlucky timing, not a result of the vaccine — the flu shot cannot give you the flu. It isn’t dangerous to get the vaccine while incubating the flu, but it may make you feel worse since your body is already fighting an infection. If you feel unwell with a fever, it’s best to delay your flu shot until you recover.

Does a vaccine for a more virulent flu strain make you feel more unwell?

No. There are a few different types of flu vaccines, but none of them contain a virus capable of infecting you. Getting the vaccine is like showing your immune system a “picture” of the influenza virus so it knows what to recognize ahead of time. It is normal to feel slightly unwell for up to 24 hours after your flu shot.

Does getting COVID boosters around the same time affect the flu vaccine’s effectiveness or increase the risk of side-effects?

No. Receiving the COVID booster and flu vaccine at the same time is safe, and both vaccines remain effective. If side-effects do occur, they are usually very mild, such as a sore arm or chills, and typically last less than 24 hours. There is no evidence that getting the vaccines together increases the risk of side-effects compared to receiving them separately.

How long does protection last, and when is the optimal time to get vaccinated?

The flu vaccine generally protects you for about three months.

Ideally, you should get vaccinated at the start of the winter season — usually in October or November — to ensure protection throughout the peak of flu season.

Even if it’s later in the season, getting the flu vaccine still provides valuable protection.

If I recently had the flu, do I still need the vaccine?

Yes. Each winter, multiple strains of influenza circulate. If you caught one strain earlier in the season, the vaccine still protects you against the others. So, even after having the flu, it is still a good idea to get vaccinated.

If someone has a compromised immune system, how effective is the vaccine, and can they still expect to receive strong protection?

People with weakened immune systems have a higher risk of getting seriously ill from the flu. Certain medical conditions or specific medications can make it harder for the immune system to produce strong antibodies.

However, immune suppression exists on a spectrum, and most people will still gain protection from the flu vaccine.

For older adults (65+), Ontario offers a high-dose flu vaccine and adjuvanted flu vaccine designed to help generate a stronger immune response.

If you are immunocompromised, it’s also important to:

• Avoid close contact with people who are sick

• Wear a mask during peak flu season

• Encourage household members and caregivers to get the flu vaccine

This adds an extra layer of protection around you.

Why should I get vaccinated against the flu this year?

While the flu shot doesn’t guarantee that you won’t catch the flu, it significantly reduces your risk of becoming seriously ill. Importantly, it also reduces the likelihood that you will pass the flu to someone more vulnerable, such as a baby, older adult, or person with a chronic illness.

How do we know what strain of flu Canada (and Ontario) is likely to get, and how well does this system work?

Canada participates in the World Health Organization (WHO) Global Influenza Surveillance and Response System, which monitors flu activity worldwide.

Here’s how it works:

1. Flu viruses are collected and analyzed from more than 100 countries.

2. Scientists track how the viruses change and which strains are spreading.

3. The WHO recommends which flu strains should be included in each year’s vaccine.

4. Canada and Ontario follow these recommendations to select the vaccine used locally.

While influenza viruses can mutate, global surveillance has been effective at predicting major circulating strains most years. Even if the match is not perfect, the vaccine still offers important protection and reduces the severity of illness.

At Yonge + Eglinton Pharmacy, you can walk in to have your COVID and Influenza vaccine, or if you prefer, you can book an online appointment

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