So WHO has just decared that the H1N1 outbreak is officially a pandemic — what does this mean? Actually very little, this is simply WHO recognising that the virus is spreading in multiple continents (its definition of a pandemic). It does not mean that the situation has currently got any worse.
However, given this announcement I thought it would be interesting to look where not to visit in the coming weeks. Below are the top ten ‘flu locations world-wide — based on how likely are you to bump into someone who is infected.
- Chile (11.6 cases per million per day over the past week)
- Austrialia (4.7 cases per million per day over the past week)
- Canada (3.9 cases per million per day over the past week)
- Panama (2.7 cases per million per day over the past week)
- Cayman Islands (2.6 cases per million per day over the past week)
- Costa Rica (1.7 cases per million per day over the past week)
- USA (1.5 cases per million per day over the past week)
- Dominican Republic (1.3 cases per million per day over the past week)
- Iceland (1.1 cases per million per day over the past week)
- Dominica (1.1 cases per million per day over the past week)
Some interesting points to note. Firstly, Mexico is not on the list, its 5th in terms of total cases per person to date, but for now the epidemic seems to be decreasing. Secondly, the top two countries are both in the Southern Hemisphere and therefore in the middle of their winter flu season — this is what we predicted over a month ago (see Blog from 2nd May). The vast majority of the others are close to the orginal source in Mexico. The one big exception in Iceland — which is very surprising.
So should we be worried? Not really. Even if you met 25 new people per day while you were on holiday for a fortnight in Chile, then your chance of contracting the infection is still less than 1:100
Cases in the UK as of 13 May 2009
The following plot shows the number of people confirmed as having been infected in the United Kingdom. So far there have been 22 identified importation events, both from the US and Mexico. The total number of cases is 71. The fact that more than half of the cases are caused by transmission within the UK means that the reproductive ratio is higher than one. This means that for each introduction, more than one further case is found. This implies that H1N1 has the potential to spread throughout the coutry.
It is important to note that there is a lag between the time of infection and case confirmation. In the next few days this picture is likely to change, as there are over 300 suspected cases being investigated by the HPA in the laboratory.
These two graphs show the total number of laboratory confirmed cases so far in several different regions, as we continue to fit simple exponential models to the cumulative data. The exceeding good news is that the rate of increase has slowed dramatically. 
Here we use two graphs to highlight the differences between different regions. The world-wide cases, fuelled by the epidemics in USA and Mexico have a steep exponential rise, with cases doubling every 5-6 days (much slower than 10th May when the doubling time was 2-3 days). In contrast, Canada, Europe and the UK show a much slower rate of increase, doubling every 9, 14 and 5 days respectively (comparable to the values from 10th May).
So does this mean that the influenza outbreak is no-longer a threat? Well, quite simply it is too early to say. The good news is that the doubling time for the number of cases has reduced dramatically in the USA; the bad news is that the number of cases is still increasing.
We now perform our 3-day prediction, based on a simple exponential fit to the recent data on laboratory confirmed cases. We also consider how well our prediction (made from data available on 11th May) performed. We now provide some measure of confidence interval based upon multiple exponential fits using different time lags and different weightings.
| Region |
True Cases 10 May |
Predicted for 13 May |
True Cases 13 May |
Predicted Cases 16 May |
| World |
3637 |
7470 |
4573 |
6360 (5856-7386) |
| excluding Mexico |
3115 |
8301 |
4051 |
5988 (5398-7308) |
| USA |
2599 |
9274 |
3436 |
5257 (4672-6790) |
| Canada |
296 |
423 |
370 |
488 (471-504) |
| Europe |
186 |
217 |
211 |
251 (236-255) |
| UK |
54 |
79 |
70 |
109 (92-113) |
One thing we are very pleased to see is that the true cases in the USAare much lower than we predicted — there has been a clear change in the growth of infection here. However cases continue to rise, and our predictions for the UK and Europe were surprisingly accurate.
These two graphs show the total number of laboratory confirmed cases so far in several different regions.
Here we use two graphs to highlight the differences between different regions. The world-wide cases, fuelled by the epidemics in USA and Mexico have a steep exponential rise, with cases doubling every 2-3 days. In contrast, Canada, Europe and the UK show a much slower rate of increase, doubling every 7, 13 and 5 days respectively.
One region that will be informative to watch over the coming days is Australia. Australia is currently in their winter (and therefore ‘flu season). Two main questions emerge: will the epidemic increase rapidly in Australia? will the new H1N1 cases be masked by cases of seasonal influenza?
We now perform our 3-day prediction, based on a simple exponential fit to the recent data on laboratory confirmed cases. We also consider how well our prediction (made from data available on 7th May) performed.
| Region |
True Cases 7 May |
Predicted for 10 May |
True Cases 10 May |
Predicted Cases 13 May |
| World |
1830 |
3009 |
3637 |
7470 |
| excluding Mexico |
1308 |
2959 |
3115 |
8301 |
| USA |
924 |
2462 |
2599 |
9274 |
| Canada |
213 |
392 |
296 |
423 |
| Europe |
149 |
299 |
186 |
217 |
| UK |
33 |
57 |
54 |
79 |
We therefore see that our predictions for the world and the world excluding Mexico were lower than the actual value, while we over-estimated the cases in Canada and Europe. However, our predictions for the UK and USA were surprisingly accurate.
The data is taken from the extremely helpful google-document compiled by the Guardian newspaper. We note that different sources provide slightly different views on the current magnitude of the epidemic. For example, looking at the total confirmed cases:
| Data source |
Worldwide cases 10th May |
Cases in USA 10th May |
| WHO |
4379 |
2254 |
| CDC |
- |
2532 |
| Guardian |
3637 |
2599 |
The graph show the current status of the epidemic (in terms of total confirmed cases to date).
The total number of cases is subdivided into several geographic regions: the world, the world excluding Mexico, the USA, Canada, Europe and the UK. Clearly the cases are increasing day-by-day and on average the number of new cases is also increasing. Mathematically the simplest way of fitting to such a pattern is with an expoential growth model — this is shown as a dashed line on the graphs.
Using this simple mathematical model, we can predict the total number of confirmed cases into the future:
| Region |
Cases 7 May |
Predicted Cases 10 May |
| World |
1830 |
3009 |
| excluding Mexico |
1308 |
2959 |
| USA |
924 |
2462 |
| Canada |
213 |
392 |
| Europe |
149 |
299 |
| UK |
33 |
57 |
These predicted values make several major assumptions: most notably that the epidemic progresses unchecked for the next three days.
Since the survey’s been live, we have had a 255 completed responses out of 368 attempted surveys. The day to day responses look like the following graph. There was an initial surge of interest, but tha has waned somewhat. It will be interesting to see what will happen once the printed surveys go out.
This is a plot of the progression of current cases of H1N1 in the world (excluding Mexico). The blue line is a simple prediction made on the 29th of April (see here). The last three points are the cases on the 30th of April, and the 1st and 2nd of May. It appears that the exponential growth has been slowed down by efforts to mitigate the impact of the epidemic in countries around the world. Nevertheless, the number of cases is still growing at a very fast rate. Data is sourced from the WHO website.
Today a further 5 cases of H1N1 flu were confirmed in the UK, bringing the total to 13. Two of those were transmission events within the UK. The other 11 are introductions from Mexico. We’re currently using this information to update our model and improve our estimates of the spread of H1N1 in the UK.
A few days ago we projected that a strongly controlled outbreak would produce around 100 cases and last around 30 days, with a 25 % probability of developing into a country wide epidemic. These projections were made using the inital conditions of the model corresponding to the known cases up until the 29th of April. If we now introduce further known cases, we are more likely to see that the infection run for longer, with around 50% of the simulations continuing for longer than 100 days, and causing more that 500 cases.
These projections are made under the assumption that no further cases will be introduced from outside the UK, which is an unlikely scenario, particularly as the US and the rest of Europe begins to have more and more cases. As more information becomes available, we will be updating this page and our projections.
