Now that the “normal” flu season is over, it seems like a good time to look back on the H1N1 influenza outbreak that first came to the public’s attention about a year ago. I’ve heard a lot of people talk about how it was all media hype and no big deal – was that the case? We’re in a position now where we can look back on the outbreak somewhat objectively.
One of the first things most people seem to look at is the number of deaths. In fact, the main argument I hear for the outbreak being overblown is that the number of deaths was pretty small. As of the end of March, the number of laboratory-confirmed H1N1 deaths in Canada was 428, with 8677 hospitalizations and 1473 ICU admissions (Public Health Agency of Canada). This only includes people who were actually tested for H1N1 though, and likely significantly underestimates the actual number. US data (from CDC) suggest approximately 12,000 dead, 265,000 hospitalizations and 59 million total cases. This compares to a previous estimate of 36,000 killed in the US each year from influenza (also from CDC). It is important to note that the estimated annual influenza deaths are not based on actual data on lab-confirmed cases, but rather on the number of death certificates listing respiratory or circulatory disease and statistical modelling, so you can’t really compare these estimates to the H1N1 lab-confirmed cases. However, it seems reasonable to take the position that H1N1 doesn’t seem to have killed more people (at least in Canada and the US) than typical seasonal influenza, and may even have killed less.
I don’t think the number of deaths is really the whole story though. When you dig into the data a bit deeper, a few anomalies appear. In particular, where the 2009 H1N1 outbreak appears to be very different from seasonal influenza is in who it affects.
Children are one group that were particularly affected. The picture below shows US pediatric deaths from H1N1 compared to historical data for seasonal flu (again from CDC, with an acknowledgment to the epidemiology blog “Effect Measure” where I first saw this):
As can be seen from this figure, pediatric mortality was way higher from H1N1 than it is from regular seasonal flu. What’s more, if you look at the raw data, there was significant mortality in children over the age of 5, where you normally don’t see much influenza mortality. Conversely, mortality was relatively low in the over 65 group, where we normally see higher influenza mortality. This is quite significant; to me there is a big difference between an elderly individual with chronic health conditions dying and a younger, reasonably healthy person dying. While the majority of those hospitalized or dying (55.9% and 82.7%, respectively) had “underlying health conditions”, the most common underlying conditions were asthma and diabetes, which are present in a lot of people.
The other group particularly affected by H1N1 was pregnant women. Canadian data show over 20% of women aged 15 to 44 admitted to hospital with H1N1 were pregnant, as were 8% of those who died. US data show 9% of the total hospitalized population (regardless of sex or age) were pregnant (for comparison, approximately 1% of the total population and about 3% of women of child-bearing age would be pregnant at any time). Unfortunately I don’t have data on seasonal flu at hand.
It’s also important to note that mortality is not the only adverse effect of influenza. The drain on hospital resources, long-term effects on people surviving severe cases of influenza, and the economic effects of high absenteeism are all significant; I’ve just focused on mortality and hospitalization rates since the data are readily available.
The other big difference between the 2009 H1N1 outbreak and normal seasonal influenza was, of course, the timing. For example, look at Canadian consultation rates:
Basically, influenza rates were far higher early in the season than normal. Later in the year (about when the normal flu season starts, and also at the time when vaccination became widespread), the rates dropped dramatically, and have since been below normal influenza rates. There has also been almost no other influenza circulating other than 2009 H1N1.
So what does all this mean? A few of my thoughts (based on the data, with a bit of speculation):
- Initial information indicated there was a potential for this flu to cause a high level of mortality, and therefore public health authorities needed to take action (can you imagine the outcry if they hadn’t taken action and it turned out to be as bad as initially feared?).
- The 2009 H1N1 virus was unusually virulent, particularly among younger people; however, the elderly (who normally represent most influenza mortality) appeared to be resistant to it. This resistance in the elderly was probably a large part of the reason why mortality rates weren’t much higher.
- By the time the normal flu season came around, a large portion of the Canadian and US population had developed resistance to the virus, either through vaccination or exposure to the virus. I suspect the proportion of the population with resistance is enough to keep the virus from returning to pandemic levels here, but not enough to wipe it out. Some parts of the US (generally where vaccination rates were lower) are currently experiencing a bit of a resurgence, so we might not be completely out of the woods yet.
Overall, my conclusion is that the 2009 H1N1 outbreak was not all hype, but thankfully turned out to be less severe than feared. The effects were still worse than normal seasonal flu, however.
There were a few positives to the situation however:
- We were able to see how public health agencies responded to a pandemic now, rather than when we there’s a really nasty one.
- A really nice randomized study was conducted in Hutterite colonies that showed vaccinating children also provided some protection to the rest of the population.
- There was a lot of increased awareness of the importance of hand-washing for the prevention of disease spread.
- We had a widespread influenza vaccine roll-out with very comprehensive safety monitoring, which in particular seems to have confirmed that influenza vaccinations do not appear to be associated with Guillain-Barre syndrome (the infamous 1977 outbreak of GBS may have been caused by bacterial contamination).
I’ve heard a lot of people, including experts, state that a major pandemic is inevitable. Personally, I’m inherently optimistic, and believe it doesn’t have to be inevitable with modern medicine and our understanding of epidemiology; however, due diligence and comprehensive action to stop any potential pandemics is necessary. Therefore, I fully support the actions taken by public health agencies to minimize the impact of the 2009 H1N1 outbreak (though not necessarily the approach taken by the media).