* The contact number is an average of the number of people to whom an infected person is estimated to transmit the infection.
* The contact number does not say anything about how many infected people there are in the population.
* If the contact number is 1.1, this means according to the calculation that 10 infected people infect an average of 11 other people.
* If the contact number is above 1, the epidemic grows while it is decreasing, if the number is below 1. If it is 1.0, the number is stable.
* The number of contacts can be calculated in several ways – for example by looking at the number of new cases of infection or new admissions. The Statens Serum Institut (SSI) does both.
* The number of contacts is retrospective – that is, they cover cases of infection that have taken place.
SSI publishes the contact number every Tuesday, but that number shows what the contact number looked like ten days ago.
This is partly due to the fact that a time shift of seven days has been included in the calculation. This is the period that is expected to pass from the time you become infected until you test positive.
In addition, the infection figures for the last three days are not used, as the figures are not complete.
* How to calculate the contact number:
The formula is: Contact number = Growth rate x Generation time + 1.
Generation time is the time that, on average, passes from one infected person becoming infected until that person infects the next. SSI estimates that it is 4.7 days. In other countries it may be different numbers – in the UK it is estimated at 6.5.
The growth rate is a figure for how the number of cases of infection develops over time. It is calculated on the basis of a complicated mathematical model, which uses the development in the number of cases of infection over the past seven days.
In that model, fluctuations in how many are tested have been taken into account and that there may be random shifts from day to day. Here it is also taken into account that from day to day in the period of illness there is a difference in how contagious you are.
* There is some uncertainty when calculating the contact number. SSI therefore has an uncertainty range.
For example, a week the contact number can be 0.6, but the uncertainty interval is 0.5-0.8 with a confidence interval of 95 percent.
That is, it is estimated that the contact number is in the range 0.5-0.8 in 95 percent of the cases.
There will thus be 5 percent of the cases where the contact number does not hit within the dial.
This is provided that the assumptions in the model are correct.
Sources: Statens Serum Institut (SSI), Viggo Andreasen, epidemiologist and RUC.
Source: The Nordic Page