Tag Archives: risk

The Age newspaper this morning is carrying a story [1] on the new Panvax ® H1N1 influenza vaccine manufactured by CSL Ltd. About two thirds of the way through the article, there is a paragraph reading

“Concerns include the development of a consent form that each recipient must sign. There is not yet enough data from trials of the drug to inform patients of the risks involved.”

It’s five days since I first wrote about the difficulties of patient consent in relation to the new vaccine so it’s pleasing to see that the issue will now get a wider airing.

There are some interesting difficulties implied in the quoted comment. The suggestion is that the information in which patients would be interested would be information that relates to the trials alone. I’m not convinced that that would be true for three reasons. Briefly, they are (i) the frequency of rare events is difficult to estimate from small samples such as the samples used in the trials, (ii) the risks of cross-contamination are largely related to human factors, and we know that human performance is frequently better when it is being watched (as in the trials) than when it is not, (iii) the population who made up the trial sample is different from the population that is likely to be first-vaccinated, and (iv) some of the adverse consequences might take a long time to show up.

Estimating the frequency of rare events

For the sake of argument, assume that the probability of a vial of vaccine being contaminated is 1 per 100,000 doses administered. If there are only 400 people in the trials then the chance that there will be an actual contamination is relatively small, and the chance of the contamination being detected is even smaller. Once a vial is contaminated and the contamination is not detected, then given a 10 dose vial and assuming a 100 percent infection rate from contaminated needles, one can expect 4–5 people to become infected. So cross-contamination might be rare, but given an occurrence the infections then cluster.

Patient differences

The people that the government is proposing to vaccinate are an unusual subset of the whole population. They include, at a higher rate than average, the people whose health or resistance has in some way already been compromised and so, I think, are more likely than the average person to have an infection of some sort. As a consequence, by estimating by the probability of cross-infection from the trial data, one is likely to under-estimate the actual risk in the mass vaccination program.

Human performance

If you are a health practitioner and you know that you are part of a study on the risks of using multi-dose vials, then you are likely to perform better than if you were just giving vaccinations without being studied. The likely change in your behaviour, which is known as the Hawthorne effect, will result in the risk of cross-infection being underestimated.

Delayed consequences

If the cross-infection that you get is Creutzfeldt–Jakob disease (CJD), then your infection is unlikely to show up for a long time and will also result in the risks being underestimated. Nonetheless, harking back to the Rogers v. Whitaker decision that I referred to in the earlier posting, I expect that patients would consider the CJD risk to be “material” even if it is very very very very small.

[1] Miller, N. (2009). Doctors urge delay on vaccine. The Age. 29 August 2009. http://www.theage.com.au/national/doctors-urge-delay-on-vaccine-20090828-f2k6.html

Contributors: Mark R. Diamond

The Australian Health Minister Nicola Roxon announced on Thursday, 20 August 2009 that, before the end of the month, the Australian government will take delivery of two million doses of the new CSL vaccine (Panvax ®) against the novel H1N1 (“Swine Flu”) virus, and that the vaccine would be supplied by CSL in multi-dose vials. The Australasian Society for Infectious Diseases reacted to the announcement by writing to the Commonwealth Chief Medical Officer, Professor Jim Bishop, urging him to delay distribution of the vaccine until it could be supplied in single dose vials. Multi-dose vials, they said, pose too great a risk of infection (as a result of contamination) to the people being vaccinated. Professor Bishop, responded by saying that the government’s pandemic plan had always included the use of multi-dose vials because they were more efficient and could be distributed more quickly.

I’ll probably return in a later posting to the matter of how one might rationally assess the risks of the proposed vaccinations. Here, I want only to highlight an interesting side issue related to patient consent to medical procedures.

In 1992, in judgement in the case of Rogers v Whitaker [1992] HCA 58; (1992) 175 CLR 479 (19 November 1992) the High Court of Australia made it clear that medical professionals have a responsibility to explain the material risks of proposed medical procedures to their patients. In particular, they said that, “a risk is material if, in the circumstances of the particular case, a reasonable person in the patient’s position, if warned of the risk, would be likely to attach significance to it or if the medical practitioner is or should reasonably be aware that the particular patient, if warned of the risk, would be likely to attach significance to it. ” It might be a cumbersome way of saying things, but it strongly suggests that if you are a doctor or a nurse about to give me a vaccination then it is incumbent upon you to explain to me, in a way that I can understand, what the material risks are of having the vaccination. It is not up to me to know all about the possible risks; nor is it up to me to formulate questions about things that I did not know I needed to know. That is the responsibility of the health professional.

It’s worth noting that, in the view of the High Court, probability per se is not of much relevance! The fact that a possible adverse consequence of treatment has only a small probability of occurring is not important in determining whether I should be told about the possibility of that consequence. Indeed, in the original trial of Whitaker v. Rogers, evidence was presented on behalf of Dr Rogers to show that the consequence that Whitaker suffered (sympathetic ophthalmia) occurred on average only once in 14,000 procedures (0.007 percent).

So, to return to the present matter of the multi-dose vials. I expect that most people being vaccinated would attach significance to the fact that multi-dose vials are more likely to lead to fatal infection by a contaminant than are single dose vials. I also think people would attach significance to each of the following: (i) that the usual Australian practice has been to use single dose vials, (ii) that any vaccinations that a person has previously had in Australia would most likely have been from single-dose vials, and (iii) that the use of multi-dose vials was part of a pandemic plan that was developed for dealing with infection by virulent avian influenza — an infection that has a 50 percent case-fatality rate as opposed to the current H1N1 virus that has a case-fatality rate of less that 0.4 percent.

I am not suggesting that every (or even any) person informed of the facts in the preceding paragraph would necessarily decide against being vaccinated. But then the High Court made it pretty clear that what I think, or what a nurse thinks, or what a doctor thinks is of no consequence when it is you who is being vaccinated. Your right is to be given the information and to make your decision in the way that you think best.

So, one can imagine the conversations that should ensue. But will they? Will Professor Bishop direct his staff to remind health-practitioners that they should warn patients about the increased risks associated with multi-dose vials?

Contributors: Mark R. Diamond