Q and A on Swine flu

View larger image


Dr. Neil Rau looks at the swine flu vaccine and why some groups are at a higher risk, such as those who are obese or pregnant, than others.

Swine flu Q and A: Who is most at risk and why?
Updated Sat. Aug. 1 2009 7:50 AM ET

Angela Mulholland and Dr. Neil Rau

While it's been four months since the world first heard of what we now know as swine flu, much remains unknown. Researchers remain puzzled about many aspects of this new virus -- most importantly, why it targets who it does. As public health officials around the world document the virus' patterns, those answers are starting to become clear. Here, CTV.ca with the help of infectious diseases expert, Dr. Neil Rau, reviews the emerging theories about how swine flu operates.

Is this a virus that primarily targets the young?
It would appear. Remember, there are still few published studies of this virus, but based on observations in North America, Europe, and now parts of the southern hemisphere, it appears that those over 50 rarely experience severe illness from this virus. Further evidence of this bug's unusual age profile as compared to seasonal flu can be found in the fact that school and camp-based outbreaks have been common while nursing home outbreaks have not been seen.

Early in this outbreak, an explanation began to emerge. Tests by the U.S. Centers for Disease Control and Prevention found that those over 50 pre-existing antibodies that protected them against swine flu, with those over the age of 60 showing the highest levels of antibodies. Again, this is consistent with the age profile of those ill enough to be admitted to hospital with swine flu infection. Less than 20 per cent of those hospitalized in the early days were over 50. By contrast, in a typical flu season, more than 90 per cent of those hospitalized are older than 50.

An additional explanation as to why those over 50 are being largely spared suggests that exposure to any previous H1N1 virus in early childhood may offer protection from future H1N1 viruses, including swine flu. Before 1957, most predominant seasonal flu viruses were weakened H1N1 descendants of the 1918 Spanish flu strain. But after the H2N2 pandemic of 1957, things changed, and non-H1N1 flu viruses became the dominant strains causing seasonal flu, "crowding out" the old H1N1 viruses from circulation. Therefore, those born after 1957 did not experience much H1N1 infection in early childhood and may not be protected from future H1N1 to the same degree as those born before 1957.

Why are pregnant patients at higher risk of death?
We still don't have the full answer, but history appears to be repeating itself. During both the 1918 Spanish flu and the 1957 Asian flu pandemics, pregnant women - especially those in the third trimester -- were at a greater risk of death.

Even during seasonal flu, pregnant women - again, especially those in the third trimester -- are far more likely to be hospitalized for complications, compared to other reproductive-age women. Pregnancy is an immune-suppressed state so that the body won't reject a "foreign" presence - i.e. the fetus. So, it makes sense that the weakened immune system is then more susceptible to defend against infections such as the flu.

As well, a woman's diaphragm in the last stages of pregnancy is pushed upward, giving her a decreased remaining lung capacity for breathing. This might allow an additional assault on the lungs, such as flu, more dangerous.

Why are the obese at higher risk of severe illness?
The most commonly cited explanation for this one is that obesity often brings accompanying chronic health conditions such as diabetes, heart disease, and respiratory problems, all of which would compromise the body's ability to fight infection. There is also the possibility that extra weight on the chest from obesity restricts the diaphragm, much as pregnancy does, making it harder to breathe and cough.

While these are plausible explanations, it should be noted that obesity has not been documented as a risk factor for severe seasonal flu.

There is an emerging new theory that obesity itself might increase susceptibility. Studies on obese mice have found the lab animals develop fewer virus-killing cells and chemicals when infected with flu, making their immune system slow to react. Not only do obese mice fail to fight off the virus, they also mount a futile, runaway immune response -- called a "cytokine storm" -- that actually kills the mice as "innocent bystanders" in the fight against the virus. It's far from clear whether the obese mouse study results apply to obese humans, but it is possible that properties of the fat cells in obese individuals inhibits their immune response and ability to fight flu.

Why are so many young people coming down with severe swine flu?
This remains the biggest mystery of this outbreak.

It could come down to the fact young healthy people comprise the largest group of people susceptible to this virus. Because this pandemic has spread most explosively in schools, young people have been exposed in disproportionate numbers. And because more young people are infected, more of the severe cases -- as well as more of the mild cases -- are occurring in this population.


What is known is that any young person with a chronic lung condition, such as asthma, can be severely sickened by swine flu. The flu makes a person's lungs more susceptible to pneumonia or other bacterial infections, a leading cause of death among those with seasonal flu. Those most vulnerable to such complications of seasonal flu are kids under two years old. Swine flu seems to mirror seasonal flu in this regard.

Are there worries swine flu could mutate to become more virulent?
No, there have been no signs of the swine flu virus mutating to cause more deaths or severe illness. Like any flu virus, this virus is expected to "drift" somewhat over time. Therefore, if it becomes established as a seasonal flu strain, future versions of vaccine may need to be adjusted. But historically, in flu pandemics, the impact of subsequent waves of a flu virus becomes weaker over time, as more people develop immunity and break the chain of spread.

The concern for the upcoming flu season in the northern hemisphere relates to the possibility that many have still not been exposed to the virus, and that even a small percentage of many vulnerable people may translate to a sizeable number of severely ill people. This did not happen during the first wave in the northern hemisphere this past spring, even in Mexico, where millions had to be infected to culminate in the number of deaths seen there.