Monday, 30 July 2012

Just A Sniff To Prevent Flu

On 25th July it was announced that the UK are to implement a program of vaccination for children aged 2-17 years old against seasonal influenza. The proposed change won’t come into effect until 2014 and it is believed that it will cost in the region of £100 million a year. There has been much talk about this level of spending with the economy the way it is, so I thought I would write a blog giving some details on the proposal and attempt to convey the benefits of the spending.

Firstly, a little background into flu vaccination. We currently have an annual flu vaccine which is given to ‘at risk’ individuals just before the onset of the flu season (winter). The vaccine contains three different strains of influenza which are predicted to be present during the up and coming flu season. Straight away we hit a slight snag here; the vaccine is made from virus strains which we are expecting to be circulating in the coming months. We need to base the vaccine on ‘guess work’ since it can take as long as six months to produce enough vaccine for everyone who needs it; this is also the reason why we only target ‘at risk’ individuals, we simply can’t make enough for everyone at present. The production of flu vaccine is slow because it relies on growing the virus in eggs, from where it will be collected and made into the vaccine. I say the vaccine is based on ‘guess work’, but this does a huge disservice to the vaccinologists and epidemiologists. The three chosen strains of the virus are picked on the basis of months of surveillance and a lot of data crunching, and the ‘guesses’ that are made regarding which strains to use are rarely incorrect. If the wrong strains are chosen, or the strains circulating change, then it we are likely to see an outbreak.

The current flu vaccine is given to over 65s, pregnant women, chronically ill patients etc. The vaccine is given as an injection and is made of inactivated virus. There are two major categories of classical vaccine, these being inactivated and live-attenuated vaccines (I’ll come back to the latter of these shortly). The inactivated vaccines contain virus particles that are completely inert; they have been ‘killed’ by treatment with chemicals. This version of the flu vaccine is injected into the arm of patients and delivers dead influenza particles which subsequently trigger our immune system to develop a, so called, ‘memory response’ against the flu virus. This memory response means that the next time a live influenza virus (of the same strain as was in the vaccine) enters the patient the immune system will have a very rapid and strong response, so as to completely protect from any severe disease.

Live-attenuated vaccines are slightly different as they do not use killed virus. These vaccines use a weakened (attenuated) form of the virus which is less capable of causing disease. An example of this attenuation is known as cold-adapted virus. To produce this form of a vaccine, virus is grown at temperatures much lower than human body temperature. The virus evolves to grow effectively at these low temperatures. When the virus is then in the human body, at a much higher temperature, it does not grow as well as it usually would. The virus will be cleared by the immune system before it is able to evolve back to growing at human body temperature and provide a protective memory response at the same time. This is just one example of how a live-attenuated virus can be produced, there are many other methods.

Both the inactivated and live-attenuated vaccines have their pros and cons, but the main point is that live-attenuated vaccines tend to give a better level of immunity since they more closely mimic a real infection, but they also carry more risk. The risk with live-attenuated vaccines comes from the possibility of ‘reversion’ whereby the virus mutates back to its normal (wild type) form. If reversion occurs then the vaccine has a chance to cause the very disease it is designed to protect against. We currently use an inactivated virus for the seasonal flu vaccine since there is no chance of reversion, making it safe to give to people who are at the most risk of a severe infection. Children tend to have much stronger immune systems than those classed as ‘at risk.’ It is therefore safer to give children a live-attenuated vaccine since they should be able to fight off the virus before it reverts and, should it revert, they are better able to recover from the disease without any severe complications.

A shot of FluMist
That covers the background, so now we can move onto the proposal of vaccinating all 2-17 year olds here in the UK. The first important point to note about the proposal is that it plans to use a vaccine that is given as a nasal spray, instead of an injection, making it much easier to give en masse (and much less painful). The nasal spray vaccine has been used in the United States since 2003, under the name FluMist, and uses live-attenuated virus.

The next big thing to consider is the reason behind targeting children with this vaccination. Flu is predominantly a disease of the old and the young, however it is much less likely that infection with influenza will cause severe disease in children, so some may ask why bother? There are a lot of reasons to bother. The main reason lies with the concept of herd immunity (which isn’t the easiest thing so bear with me). In an ideal world everyone would be vaccinated against every disease possible. If this was the case, then the virus would never cause any infections (and would die out). However, it is impossible to get 100% coverage of a vaccine in the real world. What is instead attempted is to achieve a level of coverage high enough that everyone is, in effect, protected. Let’s say there are a group of 1 million people. If 90% of these are vaccinated there are only 100,000 of the 1 million who can get the disease. If one of these ‘at risk’ individuals gets infected the chances of them meeting one of the other 99,999 who could get the disease is very slim (99,999 who could get infected against another 900,000 who can’t), making spread unlikely. Those people who haven’t been vaccinated are therefore protected simply because such a high proportion of the population has been. With a high level of coverage there is a good level of herd immunity.

If we vaccinate all children aged 2-17 we reduce the number of people who can become infected and subsequently spread disease. This will help to protect people who are at even greater risk of severe disease, for instance, a child’s grandparents who they may visit while carrying infective virus. If you have read any reports about this new proposal you may have seen the quote that, “even with moderate uptake of 30% it's estimated that this should result in 11,000 fewer hospitalisations and 2,000 fewer deaths each year,” from the Chief medical officer Professor Dame Sally Davies. These estimated figures are based around the concept of herd immunity, as well as the direct effects of vaccinating the children.

Herd immunity is not the only reason to vaccinate children. A child who gets flu will have to take time of school, potentially up to a couple of weeks. This means a parent would have to take time off work. Being at home with a sick child also makes the parent more likely to contract the disease themselves and have to take further time off work. Missing work is clearly not good for the parent, nor is it good for the economy as a whole. I have only used the examples of reduced hospitalisation and reduced time off work, but hopefully already you can see the economic logic behind the proposed move. I’d hate to have been the person to do these sums, but it has been estimated that the approximate £100 million a year cost for the vaccine campaign is cost effective when compared to the other economic benefits it could have.

For the majority of children, influenza is not a killer disease. However, starting in 2014 it is highly likely that we will see a huge campaign to get as many 2-17 year olds vaccinated against the virus as possible. The vaccine will help to reduce the cases of influenza infection in children, which, while rarely fatal, is still fairly nasty and not one you’d want to have if you can avoid it. The campaign will also help to protect those at higher risk of severe influenza by increasing the level of herd immunity. It won’t be a cheap campaign, but the simple fact that it will help prevent many cases of flu will help reduce the costs needed to treat these people, making it cost effective. Away from cost-effectiveness, if you could prevent yourself getting a nasty disease, you are at risk from, by simply having something sprayed up your nose, wouldn’t you want to? 

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