Why can you still catch the flu if you have been vaccinated against the flu?

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

Suddenly major flu epidemics are happening across the country, catching many people off guard.

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

But vaccine protection varies each year depending on how well the vaccine matches the pathogenic influenza viruses circulating at any given time. Vaccine effectiveness – a real-world measure based on the proportion of vaccinated people who still develop influenza – ranges of 16% at 60%.

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

Why it’s hard to predict which subtypes will dominate

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

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

Within each influenza A subtype, additional genetic variation can occur, with mutations (called genetic drift) generating numerous viral variants which are categorized into ‘clades’ and subclades.

H3N2 is particularly good at generating great diversity in this way. It is therefore particularly difficult to predict exactly which H3N2 virus to target in the vaccine.

Read more:
Should I get vaccinated against the 2022 flu? And how effective is it?

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

There is no guarantee that a different H3N2 virus that is not so well targeted by the vaccine will not arrive in the country in the months leading up to winter and start causing disease.

The subtypes contained in the seasonal flu vaccine are selected months in advance.

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

With the easing of travel restrictions towards the end of 2021, cases of influenza 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 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 efficacy of only 16% protection against mild to moderate H3N2 disease. Protection against more serious diseases was only 14%.

We don’t know what subtypes will be circulating in Australia

Data on influenza vaccine effectiveness in the Southern Hemisphere winter of 2022 are not yet available, and it is unclear how well the current vaccine protects against currently circulating pathogen subtypes.

While H3N2 viruses appear to be drive a disease nowother influenza viruses may become more common later in the season.

The influenza vaccine is a quadrivalent vaccine, so in addition to influenza A H3N2, it will protect against another subtype of influenza A (H1N1) and two separate lineages of influenza B virus. These viruses do not change as quickly as H3N2, so the vaccine is more likely to provide better protection against these other influenza viruses.

Although vaccine protection against H3N2 is weaker than usual this year, the vaccine could make the difference between recovering at home and ending up in the hospital.

Read more:
Which flu vaccine should I choose? And what are cell-based and “adjuvanted” vaccines?

So who should get the flu shot and when?

The flu vaccine provides the highest level of protection during the first three to four months after vaccination. The season usually 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 end. So it’s not too late to get vaccinated.

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

  • Aboriginals and Torres Strait Islanders six months and older
  • children aged six months to five years
  • pregnant women
  • people aged 65 or over
  • people aged six months or older who have medical conditions that mean they have a higher risk of getting a serious illness.

What if you still catch the flu?

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

Specific antivirals for influenza can help, if given early. To ensure rapid access to particularly vulnerable elderly residents, elderly care facilities are stocked with the flu antiviral drug Tamiflu.

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

#catch #flu #vaccinated #flu

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