Why there are conflicting numbers on the flu shot efficacy

The death of two children from the flu in Guelph has left friends and family devastated and many more wondering:How effective is the flu shot?

Dr. Mark Loeb, an infectious disease expert with the department of medicine at McMaster University, told CBC's The Morning Editioncomponentsof influenza A an B are in every flu vaccine.

Some vaccines have two types of influenza A and one type of influenza B, and a newerquadravalentvaccinehastwo types of each.

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He said ideally thereneeds to be a match between the flustrains, the vaccine and the fluthat will eventually circulate among peoplefor the vaccine to be fully effective. But that process is not always perfect.

"The challenge however, is that the vaccine needs to be produced months before the flu season," Loebexplained. "So scientists will make the best prediction as to which strains to put in that vaccine."

So how effective is it?

Some reports suggest this year's vaccine is only 10 or 20 per cent effective against this year's flu strain.

Loebsaid it's challenging to pinpoint how effective the flu shot is in preventing the flu because it depends on what strain is being discussed.

"The match this year between what's in the vaccine and what's circulating is pretty good on paper, butsometimes in the manufacturing process, there's changes that occur even in to the strain that is made in the vaccine and that can cause a difference," he said.

Not all of this year's flu data has been collected, he said, but early estimates of effectiveness for this year's strain of influenza A suggests it sits around 10 to 20 per cent and 55 per cent for influenza B.

He said it's important for children vaccinated against the flu, but stopped short ofrecommending making the flu shot mandatory, like measles mumps and rubella.

"Even when it's not perfect, the flu vaccine still offers a level of protection,"Loebsaid.

More flu clinics

Wellington Dufferin GuelphPublic Health has added new flu clinicsfor the rest of this week following the widespread concern after two children from the same school in Guelph died from influenza B.

Public health has told CBCNews it tracks thenumber of schools with greater than 10 per cent of students absent on a weekly basis.

That number peaked two weeks ago with nine of 12 reporting schools said to have had over 10 per cent student absence.