In the book, Health Cheque, Geoff and I found that in terms of health, prevention is more effective than cure by a factor of four to one. So the Morgan Foundation retains an interest in the issue of prevention, which led to further work on food & nutrition (Appetite for Destruction). Our new researcher Dr Jess Berentson-Shaw has a health background so we thought we’d introduce her with a topic that she has some expertise and is something we get a lot of questions about: immunisation.

In two blogs, Jess will ask:

1) Should you get your kids immunised? and

2) What is the status of immunisation in New Zealand? Is anyone losing out?

These blogs also serve as an introduction to some of the research Jess is doing exploring why some kids and families don’t do so well in this country and what the evidence says is the best thing to help them out.

Widespread immunisation helps everyone

A vaccination prevents an individual getting a disease, that much is pretty straightforward . However, widespread immunisation also has benefits. If enough people are vaccinated it eventually eliminates a communicable disease (a disease you can catch from someone else who has the virus) in a community, a society and eventually worldwide. Smallpox for example is eradicated worldwide.

This is known as “herd immunity”: when enough people are fully immunised (for most vaccines this is near 95% of the population), the risk of someone catching the disease and passing it to someone else not immunised is very low. Eventually the disease cannot continue to live as it cannot find enough hosts for it to breed in, and it becomes extinct. If only we could do that with rats and possums so effectively!

But a vaccination comes with risks

No medical treatment is without risks. Having an ingrown toenail removed has a risk of toe amputation, having a general anaesthetic has a risk of brain damage.

But with most interventions the likelihood of the most severe side effect is much smaller than the risks associated with not doing the thing. So there are some risks when a child is immunised, for example inflammation at the site of the injection or fever. There are even smaller risks of some more serious side effects like an allergic reaction called anaphylaxis. However, such severe side effects occur much less often with the vaccine than they would if a person caught the disease itself.

The figure below compares the risks of having the MMR (Measles Mumps Rubella) vaccine vs taking your risks with measles. If a million children had the vaccine, and another million caught the disease, then we would expect to see the numbers of complications in the table below.

MMR Vaccine	Measles
Uncommon Complications
300 children have seizures	10,000 children have seizures induced by fever
Rare Complications
26 Children bruise or bleed more easily (thrombocytopenia)	330 Children develop thrombocytopenia
VERY Rare Complications
Up to 4 children get severe anaphylaxis (allergic reaction)- treatable	0 Children get anaphylaxis
0 Children get SSPE (causes progressive brain damage & death)	10 Children get SSPE
Up to 1 child may develop encephalitis (brain inflammation that can cause brain damage and death)	2000 Children develop encephalitis

 

Figure 1. Severe complications due to MMR vaccine and measles among 1 million children aged under 5 years. Source:  The Australian Academy of Science.

What we can say is that there is NO risk what so ever that you will get autism from a vaccine.

This creates a prisoner’s dilemma

This combination of factors gives rise to a prisoner’s dilemma. A what? You can find out more about the classic prisoner’s dilemma (and why it involves prisoners) here. For now we will stick with the immunisation theme.

As as immunisation rates grow in a society (as they are currently in New Zealand), the risks of contracting a disease lessen,  as do the overall risks in a population of being hospitalised or dying from that disease. So if enough other people are immunised, then it might be rational for some people to take the risk and and choose not to vaccinate their child. In other words, if the chances of their child contracting the disease are low, the parents might choose to avoid the (also low) risks associated with getting the vaccination.

Example of a Prisoners Dilemma

For simplicity, let us pretend a community has two undecided and unrelated parents, Andrew and Brian. They have herd immunity explained to them, alongside the risks of the vaccination for measles, the risks of contracting the disease and the risks of hospitalisation or death if their child gets the disease.  Each are then asked to decide whether to vaccinate their child against measles. The risks and rewards for them are as follows:

• If Andrew and Brian both don’t vaccinate, each child has a higher risk of contracting the disease and of experiencing serious health effects of the illness, but they avoid the risk of vaccination side effects.
• If Andrew does not vaccinate but Brian does, Andrew’s child will have a lower risk of catching the disease, low risk of experiencing serious health effects of the illness and no risk of vaccination side effects. Brian’s child will have a low risk of the disease, low risk of experiencing serious health effects of the illness and some risk of vaccination side effects (and vice versa). In short, Andrew is taking a ‘free ride’, benefitting from Brian’s choice to immunise his child.
• If Andrew and Brian both vaccinated, both children will have the lowest risk (becoming no risk) of the disease and experiencing serious health effects of the illness, while both children have a risk of vaccination side effects.

Assuming Brian vaccinates his child, it might make sense for Andrew to choose not to. And this applies in the real world – in a mathematical sense the truly rational parent (when understanding the real scientific risks, we are not talking myths here) may choose not to vaccinate, provided that enough of society has been vaccinated to provide herd immunity. In this situation, a rational parent could view the risks of vaccinating their child as much greater than the risk of their child contracting the disease.

Why the ‘Free-Riders’ are hurting their kids (and others) in the long term.

You can probably see the problems in this ‘rational’ but ultimately selfish decision process, and not just for the wider community as a whole. In taking this ‘free ride’, parents who chose not to vaccinate could be harming their child in the longer term. As the vaccination rates decline (as more people choose not to vaccinate believing the risk to be low) the actual risks of contracting the disease then rise. So every child is now exposed to a greater level of risk, BUT especially those that are unvaccinated.

How do we know this? Because in countries with low vaccination rates, their rate of disease is at epidemic proportions. This interactive map shows where measles is at an epidemic rate worldwide – the majority of deaths from measles are in children under 5. Afghanistan which has an immunisation rate of less than 40% had 6000 cases in 2012 (likely to be more due to poor reporting mechanisms).

And just in case you think this is a developing world problem, in New Zealand in 1991 our immunisation coverage rate at 2 years was less than 60% overall, and only 42% In Maori and 45% Pacific children . In 1997 we had a large scale measles outbreak, there were 2169 cases notified, near 100 people hospitalised and 7 people died, four of those who died were children who were not immunised(Ministry of Health Immunisation Handbook, 2014). As we will see tomorrow, even now we don’t have the levels of immunisation we need for herd immunity – so any parent not immunising their child is rolling the dice with disease.

If an unimmunised child does come in contact with measles for example, there is a 90% chance they will get it, and if they do get it there is a 1 in 5 chance they will be hospitalised for serious complications and a 1 in 1000 chance they will die. So if increasing numbers of parents choose not to immunise due to a low risk of disease and a comparatively higher risk of vaccination side-effects they are actually, ironically, increasing the risk of their child becoming seriously ill during outbreaks. What is almost worse is that they are putting some already really vulnerable kids at greater risk.

Free-Riders risk making sick & vulnerable kids sicker

For children with compromised immunity, and in New Zealand this is mainly kids receiving chemotherapy treatment for cancer, if they contract measles they have a 1 in 2 chance of dying from it. Young babies, who have undeveloped immune systems and are too young to get a vaccination, are at high risk of contraction and hospitalisation during an outbreak of a disease. Kids in poverty who already suffer an additional disease burden compared to their better off counter parts, are more vulnerable also due to their lower vaccination rates (an issue we will go into in the next blog).  So as the rate of a disease increases due to a decline in vaccination rates, the kids that suffer from this decline are those that are least able to cope.

Only a few of us can have a free-ride and it needs to be based on need.

If herd immunity is achieved when 90-95% of the population are fully immunised (depending on the disease) and this rate is maintained for a period of time,  then the 5-10% we can carry unimmunized in our society needs to be reserved for those with the greatest need, not those who want to opt out due to illogical perceptions of risk. It helps if information about the risks and benefits, both individual and population based, are communicated effectively and people are given the opportunity to understand that their personal decision affects everyone.

Summary

In summary, having your child vaccinated helps not only your child but everyone in society. It is theoretically plausible that if enough people have their child vaccinated, it might be rational to not have your child vaccinated, given the low risk of side effects. But the more people that make that decision, the greater the risk of an outbreak becomes. And that can potentially hurt anyone that isn’t immunised. The next blog will look at the situation in New Zealand, and will expand on why the poor and sick are the ones that lose out when people choose not to be immunised.

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