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From which flu strain will be dominant to how effective this year's vaccine will be, take a deeper look at the key factors experts are monitoring

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It's that time of year again when clinics and pharmacies offer flu shots, hospitals get ready for a surge of sick patients, and caregivers of young children and the elderly brace themselves for a winter of coughs, fevers and sniffles.

Just how nasty will the coming flu season be? It's a question that eludes even the experts. Since flu viruses are constantly changing, the severity and timing of each season are notoriously unpredictable.

"I've been in this business long enough to know that it is treacherous to make influenza predictions," says Danuta Skowronski, epidemiology lead for influenza and emerging respiratory pathogens at the BC Centre for Disease Control, who has been studying the flu for about 20 years.

Yet even though they warn it's too early to tell how the season will pan out, experts like Skowronski are keeping a close eye on a few key factors, including signs of which flu strain will be most prevalent, and how effective this year's flu shot will be.

Here's what to watch for:

Which flu strain will be predominant?

Influenza viruses are grouped into several different types, but when referring to seasonal flu, it's the A and B types that are the main culprits. For influenza A, the subtypes that public health experts are monitoring closely are H3N2 and H1N1.

These two kinds of influenza A viruses have very different profiles, Skowronski says. Flu seasons in which H3N2 is predominant tend to be more severe, characterized by a greater number of hospitalizations and deaths.

H3N2 also tends to hit seniors hardest, whereas in H1N1 seasons, more children and younger adults are typically affected, and there are generally fewer hospitalizations and fewer deaths, says Bryna Warshawsky, medical director of communicable disease, emergency preparedness and response at Public Health Ontario.

What we have experienced over the past few years is an alternating pattern in influenza A subtypes, Warshawsky says. Last year was an H3N2 season, and the year before was an H1N1 season. That was preceded by an H3N2-dominant flu season and an H1N1 season the year prior to that, she says.

If this pattern holds, we could be in for a milder H1N1 flu season. The trouble is there's no telling whether the pattern will persist.

video A microbiologist shares her tips for avoiding the flu this season

What will be the fallout from the summer flu season in the southern hemisphere?

Countries in the southern hemisphere, including Australia and South Africa, have had a relatively severe flu season due to H3N2 over the summer months, Skowronski says. Hong Kong has also had a particularly long and deadly outbreak. Based on what's been happening in other parts of the globe, there's been much speculation that Canada will likely experience a severe epidemic, too.

But Skowronski says many are overlooking the fact that Canada already had a fairly severe H3N2 epidemic last winter. "So the question is: Are [the other countries] following us? Or will we be following them?" she says.

Either way, a big question is whether we have sufficient immunity among our population to keep whatever constantly changing flu variants are circulating at bay, she says.

Warshawsky says it's difficult to make solid predictions for our flu season based on the experience of other countries, since they vary from country to country. Australia, for instance, she says, was hit by influenza A and influenza B at the same time, which created a large burden on its health-care system, whereas the two influenza types don't typically overlap in Canada. "We usually tend to see influenza A first, and influenza B later," she says. "So all of that makes what happened in other countries hard to figure out how that might impact us now."

How effective will this year's flu vaccine be?

Every year, around February or March, the World Health Organization provides its recommendations on the composition of influenza vaccines for the northern hemisphere for the next flu season, based on its projections of what viruses are likely to be in circulation. But it's hard to predict just how effective the vaccines will be.

In general, flu vaccines are around 50 per cent effective. But for the 2014-15 season, the vaccine effectiveness against H3N2 was less than 10 per cent. Flu shots are by no means perfect, but they're still considered the best way of protecting people from getting sick.

The trivalent flu vaccine given this year, which contains three components, is comprised of an H1N1 vaccine component, an H3N2 component, and an influenza B component.

While the H1N1 component in this year's flu shot has been updated for the coming season, the other two components have remained unchanged from last year's flu vaccine, Skowronski says. Depending on which is the dominant strain this year, this could spell trouble.

"If it turns out to be a H3N2 season, then that means the vaccine effectiveness is likely to be suboptimal," she says. That's because last year, with the identical component, the vaccine effectiveness for H3N2 was around 35 to 40 per cent. And since the viruses are constantly changing and mutating, Skowronski says it's unlikely the effectiveness of the same vaccine component will be any higher for the coming season. "That's one of the unfortunate, concerning factors, frankly, from my perspective: that the H3N2 component is unchanged, yet we know the virus is changing."

Even so, just because this year's flu shot contains two out of three of the same components as last year's, don't think you won't need to get vaccinated again if you got the shot last year. The updated influenza A component may help protect you in an influenza A outbreak, Warshawsky says. Plus, she adds, "We also know that the duration of protection doesn't necessarily last well from one year to another. So relying on last year's vaccine will not necessarily carry over protection to this year."

What effect do previous flu vaccinations have on the current flu shot?

It's a subject of debate, but there may be some reason to suggest that getting flu vaccines year after year could reduce their effectiveness. Skowronski says emerging data appears to indicate flu vaccines may not work as well in people who have had repeat flu shots in previous years, compared with those vaccinated for the first time.

Since these findings are still emerging, researchers don't have any conclusive information about the mechanisms to explain why this might be, Skowronski says. But one hypothesis is that a pre-existing antibody in individuals who've had previous flu shots may be "mopping up" or interfering with the antigens that they get in the current season's flu shot, she says. It could be that a new flu shot somehow refocuses a pre-existing antibody to target older antigens rather than current antigens.

"If it's a real phenomenon, that repeat immunization effect may play a role this year with [the] unchanged H3N2 vaccine component," she says. If it turns out to be a H3N2 season, those who received last year's flu shot may end up being less protected by this year's vaccine.

Even so, there's not yet enough evidence about this possible effect to change public health policy, Skowronski says. And she emphasizes in most seasons, flu shots still provide better protection to those who've had prior flu vaccinations than those who aren't vaccinated at all. So you're still better off getting the flu shot than not, even if you were vaccinated last year.

Some researchers dispute whether the effect is real in the first place. Warshawsky and her colleagues conducted a systematic review and meta-analysis, published earlier this year in the journal BMC Medicine, that found no evidence that prior flu shots lower the efficacy of vaccinations. She says differences in research methods could explain why some studies have found otherwise.

Either way, researchers will be tracking the efficacy of this year's flu vaccine, which may help contribute to their understanding of what's going on.

WATCH A day-by-day look at how the flu infects your body

What role do vaccination rates play?

When it comes to the flu vaccine, it's difficult to achieve herd immunity, which is the protective effect of having a large proportion of the population immunized against a contagious illness, Warshawsky says.

Unlike other vaccines, like the measles, mumps and rubella vaccine (MMR), which is about 97 per cent effective at preventing measles after two doses, the flu vaccine is typically only about 50 per cent effective. It also must be given annually. Plus, every year, only around 30 per cent of the population typically gets vaccinated for the flu.

What's important to note is if you don't get the flu, you won't pass it on to anyone else, Warshawsky says, so on an individual level, protecting yourself with a flu shot helps protect those around you. "But on a population level, to get enough people vaccinated to actually stop circulation [of a flu virus], that's going to be harder to achieve."

What's holding people back from getting vaccinated?

Vinita Dubey, associate medical officer of health at Toronto Public Health, cites what's known as the "three Cs": confidence, which involves people's level of trust in vaccines, in vaccine manufacturers, heath-care providers and the government; complacency, which involves a lack of understanding about the need for a vaccine or ambivalence about getting vaccinated; and convenience, or the ease of getting vaccinated.

People who get flu shots tend to be motivated not just to protect themselves, but also to protect others, such as family members, Dubey says.

To encourage more people to get their annual flu shots, she says public health officials try to target people at the community, school or childcare centre level, for example, by recommending that childcare workers get the flu shot not just for themselves, but for the children under their care, particularly those under age 5 who are at high risk.

At the organizational or institutional level, she says, some workplaces, such as hospitals, have policies that require workers to get the flu shot or wear a mask during flu season. These measures likely affect people's decision to get vaccinated, she says, noting that Toronto Public Health posts the influenza vaccination rates of certain workplaces, such as health care facilities, to encourage those facilities to urge their staff to get their flu shots.

Dubey says Health Canada data suggests offering flu shots at pharmacies may contribute to a slight increase in the number of people getting vaccinated. "So being able to offer it in different places is helpful."

Should children receive the nasal spray flu vaccine?

For children, the nasal spray FluMist has been offered as a pain-free way to receive the flu vaccine. But there are conflicting recommendations about whether to use it.

The U.S. Centers for Disease Control's advisory committee on influenza practices has recommended against using the nasal spray, which contains live attenuated influenza, due to concerns about its effectiveness. Alberta Health Services announced earlier this year it would not make FluMist available through the provincially funded influenza program this season. And Saskatchewan public health clinics won't be offering the nasal spray.

Bryna Warshawsky points out Canada's National Advisory Committee on Immunization – which sets the vaccination recommendations for the country – says that for children aged 2 to 17, either a flu shot or nasal spray can be used.

The mixed messages about the nasal spray vaccine stem largely from conflicting U.S. and Canadian data. Canadian and some European study results show somewhat higher rates of effectiveness for the nasal spray than U.S. results, says Warshawsky, medical director of communicable disease, emergency preparedness and response at Public Health Ontario. But the reason for the difference is unclear. "People don't really know," she says.

"It's really a matter of speaking to your health-care provider to decide if there's any reasons that you shouldn't get one or the other … and then the preferences of the parents and the child," she says.