Monday, 22 July 2013

Vaccination works!

Perhaps not such a surprising headline; but it is nice to see it demonstrated.

One of the wonderful things about vaccination is that as an intervention it works at multiple levels, most notably at the individual and popluation levels.  What that means is that when you vaccinate an individual, as long as that vaccine has worked they should be protected against infection - that is the individual level protection.  Furthermore, because fewer people are catching or carrying the disease; it does not spread so much within the population, giving some degree of protection to people who are not immune.  This is a concept known as herd immunity, and it is particularly powerful when vaccination coverage is high.  This is based on a number known as R0, which I discuss a little here, in summary to find the crucial proportion needed to achieve herd immunity the calculation is 1-1/R0 (so if the R0 is 10, the cacluation is 1-1/10 or 0.9 or 90%.

So if you are caring for someone who is particularly susceptibe to infectous disease, one of the best things you can do is to ensure that you are fully up-to-date with your vaccines.  This is one of the rationales for vaccinating children against influenza, clearly to protect them, but also to protect grandparents and others at risk of severe influenza illness.

A study from America has looked at this phenomena with regards to hospitalisation with pneumonia (Griffn et al (2013) N Engl J Med 369 155-163).  It looked at three periods: before the introduction of the pneumococcal vaccination into the infant schedule; its early days of introduction; and a latter phase when it was fully embedded.  The findings were stark, among both young and old the numbers of people admitted to hospital with pneumonia was reduced, from period one to three the reduction was 43.2% in those under 2 years of age (who would have had the vaccine) but even more impressively by13% in those aged 75-84 years and by 22.8% in those over the age of 85 years.  Overall the reduction ws 10.5%, in numbers this equated to a reduction of 168 000 hospital admissions in one year.  (When you see percentages or relative risks/odds ratios always look at the actual numbers as well - remember 10.5% of nothing is.....nothing.  These numbers are fairly impressive though).

Now there are loads of issues around these findings: just two of the questions are what else might have happended during this period (they discuss reductions in smoking and influnza vaccines for example) and how reliable are the data?  But however cynical one might be, these are impressive results.  The crucial thing to remember though is that herd immunity, which is what is being seen here, relies on high vaccination levels.

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