Influenza vaccine effectiveness in preventing hospitalizations

An Israeli paper (PMID 30753347) published in 2019 in the very prestigious journal “clinical infectious diseases”. This is an epidemiologic retrospective study based on a huge database from Clalit health services, one of the largest healthcare services providers in the world.
The study’s question was the effectiveness of influenza vaccine in preventing hospitalizations in children due to influenza.

Introduction: it is known that children are at high risk for influenza infection and its complications (hospitalizations included). A vaccine is currently the only effective means in preventing influenza, and it is recommended in Israel before every winter season for children above 6 months.
Due to the fact that the dominant strain of influenza varies from one season to another, and due to the fact that the effectiveness of the vaccine is limited, there is a question whether the vaccine in children (a single or dual dose for children younger than 9 years that got 0-1 doses in previous seasons) prevents hospitalizations (or in other words – severe influenza infection, with or without complications).

Methods: data from 3 seasons were collected: 2015-2016, 2016-2017, and 2017-2018. In a very intuitive manner, the rate of hospitalizations in children from 6 months-9 years were compared between those vaccinated (one or two doses) and those who weren’t.
Excluded from the study were those who were vaccinated less than 14 days before hospitalization, those that were vaccinated by a nasal spray (which was only available for some part of the study’s’ time period), or those with a positive test for influenza more than 10 days after hospitalization (those are thought to have contracted the virus during their hospital stay)
How did they determine a child had contracted influenza? By performing a PCR test for influenza.
Data of 3147 children, hospitalized in 6 different hospitalized (326 of them positive for influenza) was analyzed.

What was the effectiveness of the vaccine in preventing hospitalization?
The effectiveness in children properly vaccinated was 53.9% (in other words, the vaccine prevented over 50% of hospitalizations). The effectiveness of the vaccine in children partially vaccinated (received just one dose instead of two doses) was 25.6%.
The effectiveness was significant through all the study’s years though there was a difference in effectiveness between the different strains (A VS. B). for example, in the season of 2015-2016, the vaccines’ effectiveness in preventing influenza A was above 80%, while effectiveness against influenza B was only 23%. In general, effectiveness was higher against the A strain than the B strain. In addition, the effectiveness of a full dose vaccine was higher than a partial vaccine (which was off course higher than not vaccinating at all…) throughout all study’s duration.
There was no difference between different age groups.

In summary:

This is a good, simple study, proving without a doubt that the influenza vaccine is effective in preventing hospitalization due to influenza infection in children younger than 9 years old.
The study also proved that there is a reason for the current recommendation of two vaccine doses in the same season, for children who were only vaccinated once before (or not vaccinated at all).
Those who are against vaccinations would say that the most important data in this study, effectiveness of 53.9% in preventing hospitalizations, is not enough. True, it is not enough, and the whole pharmaceutical community is at a race trying to find a more effective vaccine. But for now – this is what we’ve got, and it is not bad at all.
Moreover, it is important to mention, that this year as appose to the research’s years, the vaccine includes 4 strains (2 A’s and 2 B’s). Since the effectiveness against influenza B was low, I think that with the new vaccine– effectiveness would go up.
You must remember that not every year the world Health Organization (WHO) is successful in predicting the strain of influenza arriving to the north hemisphere. You can see that when the prediction is more accurate, the effectiveness of the vaccine is higher (and vise-versa).
And one last thing – remember all of those children that are hospitalized due to influenza infection. They can be children with or without underlying illnesses, with just respiratory distress, who needs only supportive care. But, some suffers from complications, such as secondary bacterial infections, encephalitis (neurological complications), myocarditis (cardiac involvement) or even death.
So If I would suggest lowering the risk for hospitalization in half – wouldn’t you take that?
So please – vaccinate your children and yourself against influenza.

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