Why can you still get influenza if you’ve had a flu shot?

Why can you still get the flu if you've had a flu shot?

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Restrictions have been relaxed, international borders are open and the flu is back in Australia after a two-year absence.

Suddenly, major flu outbreaks are spreading across the country, taking many by surprise.

Influenza vaccinations aim to protect against four influenza viruses that cause disease in humans (two subtypes of influenza A and two subtypes of influenza B).

But vaccine-mediated protection varies each year, depending on how well the vaccine matches the disease-causing flu viruses circulating at any given time. Vaccine effectiveness – a real measure based on the percentage of vaccinated people who still get the flu – ranges from 16% to 60%.

However, it is still important to get your flu shot. If you have been vaccinated and still get the flu, you are less likely to get that sick.

Why it’s hard to predict which subtypes will dominate

Of the four types of influenza viruses that occur in nature, two cause significant disease in humans: influenza A and influenza B.

The 2022 flu vaccine is quadrivalent (targets four different viruses): two influenza A viruses (subtypes H3N2 and H1N1) and two influenza B viruses from different lineages.

Within each flu A subtype, further genetic variation can arise, with mutations (known as genetic drift) generating many viral variants classified into “clades” and sub-clades.

H3N2 is particularly good at generating a lot of diversity in this way. So it is very difficult to predict exactly which H3N2 virus the vaccine should target.

A major challenge for flu vaccines is the decision on which virus to target must be made months in advance. The H3N2 virus in the Australian flu vaccine (A/Darwin/9/2021) was selected in September 2021 to enable production and distribution of the vaccine in time for the winter of 2022.

There is no guarantee that another H3N2 virus not controlled as well by the vaccine will not arrive in the country in the months leading up to winter and cause illness.

Another factor that has made predicting which H3N2 virus in the vaccine for 2022 particularly difficult is the lack of data on which viruses have been dominant in previous flu seasons, both in Australia and across the equator.

With the easing of travel restrictions towards the end of 2021, flu cases began to reappear during the winter of 2021-22 in the Northern Hemisphere. But the lack of flu cases in previous seasons (due to COVID) meant that the data used to predict which viruses to target was inadequate.

The US Centers for Disease Control (CDC) analyzed data from more than 3,000 children and found a vaccine effectiveness of only 16% in protecting against mild to moderate disease from H3N2. Protection against more serious diseases was only 14%.

We don’t know which subtypes will circulate in Australia

Data on the effectiveness of flu vaccines in the southern hemisphere in the winter of 2022 is not yet available and it is unclear how protective the current vaccine is against the currently circulating disease-causing subtypes.

While H3N2 viruses seem to cause illness now, other flu viruses may become more common later in the season.

The flu vaccine is a quadrivalent vaccine, so in addition to influenza A H3N2, it will also protect against another influenza A subtype (H1N1) and two different lines of the influenza B virus. These viruses don’t change as quickly as H3N2, so the vaccine is more likely to provide better protection against these other flu viruses.

Even though vaccine protection against H3N2 is lower than normal this year, the vaccine could make the difference between recovering at home or ending up in the hospital.

So who should get a flu shot and when?

The flu vaccine offers the highest protection in the first three to four months after vaccination. The season generally peaks between June and September, although this year we have seen a much earlier than usual start to the flu season. It’s unclear whether this early start will mean a longer flu season or an early ending. So it’s not too late to get vaccinated.

Flu vaccines are recommended for anyone six months and older, but are especially important for people who are more at risk for complications from the flu, including:

  • Aboriginal and Torres Strait Islanders aged six months and older
  • children from six months to five years
  • pregnant woman
  • people aged 65 or older
  • people six months of age or older who have medical conditions that put them at higher risk of developing a serious illness.

What if you still get the flu?

If you develop flu symptoms, isolate yourself and see your GP for a flu PCR test to determine if you are indeed infected with the flu, especially if you are in the high-risk groups.

Specific antivirals for the flu can help, if given early. To ensure rapid access to particularly frail elderly people in aged care, aged care facilities are filled with the antiviral flu drug Tamiflu.

In New South Wales, free drive-through clinics are now offering tests for influenza, respiratory syncytial virus (RSV) and SARS-CoV-2, the virus that causes COVID. Other states and territories may follow.


Flu vaccine may not protect against main flu strain: Study


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