Some H1N1 math

It’s making me a bit crazy that people are avoiding the H1N1 vaccination. You’d rather be flat on your back for a week with fever, cough, body aches and a bad headache than have one small needle-stick and a vaguely sore arm for a day? Well, to each his own. But let’s look at the relative risks.

First, let’s do some math, just based on the known numbers.

As of yesterday, there have been 95 deaths due to H1N1 in Canada, and roughly 1600 hospitalizations. How can we estimate what our personal risk might be?

Well, that’s tricky, because they stopped reporting the number of actual cases of flu some time ago. So for the moment, let’s assume everyone — all thirty-four million of us in Canada — were exposed and got sick. That’s the most conservative assumption we can make when we’re calculating odds about how many people have died, right? That so far we’ve all been exposed and only ninety-five people have died?

A one in a million chance of dying would then have caused 34 deaths across the country, but there have in fact been 95 deaths, so in this case your odds of dying would be roughly three in one million. Your odds of being hospitalized are about sixteen times that (1600 vs. 95), if we continue along the same trajectory of illness, so about 48 in one million.

Of course, we know the actual rate must be higher, because NOT everyone has been exposed or has become sick, and yet we still have those 95 deaths: the 95 deaths have come from a smaller number of people than thirty-four million. So let’s use a less-conservative estimate, one that’s currently being proposed as reasonably likely, and say a quarter of the population — 25% of 34 million, so 8.5 million people — get sick, and we still have those 95 deaths. Then what are your odds? Basic math puts it at about one in 90,000, or about eleven in one million. And your odds of being hospitalized are still about sixteen times that, 176 in a million, or 1 in about 5,700.

We know the true odds are higher for two reasons. One, not even a quarter of the population of Canada has become sick yet, and we still have those 95 deaths and 1600 hospitalizations, so the true odds of death or hospitalization even now are higher than my calculations above. Two, we know more people will be hospitalized and more people will die, again raising the odds, since people are still getting sick.

But for argument’s sake, let’s just use the eleven-in-a-million number, even though we know it’s too low.

OK, now on to the vaccine.

Serious reactions to flu vaccines are normally about a 1 in 100,000 occurrence. We can reasonably assume this vaccine will be similar, since it’s been made in exactly the same way as previous vaccines have been made: you pick the strain, you grow it in eggs, etc.

Sometimes really strange reactions happen, like dystonia (which can also be caused, as I understand it, by the flu itself, and is a one-in-a-million kind of thing) or Guillain-Barré Syndrome (although getting the flu shot does not increase your odds of contracting Guillain-Barré generally, and is also about a 1-in-a-million risk).

Sometimes the flu vaccine is how people discover they’re allergic to eggs (since it’s grown in eggs) and they go into anaphylactic shock. This is apparently one of the most common bad reactions, and it is why responsible clinics will make you sit there for ten or fifteen minutes after you get the shot — they’re making sure you’re not going to go anaphylactic on them. But it is extremely, extremely rare for people to die from getting a flu shot.

The complication numbers here are pretty solid, known numbers; we’re not pulling them out of the air as we are in calculating death & hospitalization rates from actual flu. I won’t get into the adjuvanted vs. non-adjuvanted issue here, but the complication rates out of Europe for adjuvanted vaccines generally seem to be comparable to the 1 in 100,000 figure I’m using here.

So getting the shot is a 10 in a million (=1 in 100,000) kind of risk. Even if you’re one of the unlucky ten, though, you’re most likely still alive. And here I will repeat that because the shot is made with dead virus, you cannot contract the flu from the vaccine.

Compare that to the chance of hospitalization if you do contract flu, which we conservatively calculated above to be 176 in a million. You’re about eighteen (remember these are low estimates) times more likely to end up in the hospital if you get the flu than if you get the shot.

Also, compare that 10 in a million risk of complication from the vaccine to the 11 in a million (again, estimating conservatively) risk of death from actually contracting the flu.

They’re comparable numbers. But which risk would you rather take? The one that leaves you alive, or the one that leaves you dead?

Get the shot.

4 thoughts on “Some H1N1 math

  1. Thank you, oh thank you Robyn for the lucid, logical analysis, something we’re sorely missing in all of the broken-telephone hysteria that seems to prevail at present.

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