|Clarification on certain disease model parameters [message #635631]
||Wed, 27 October 2010 14:48
| Traci Arthur-Hartranft
Registered: August 2010
Hi - Can you help me with some clarification on specification of transmission rate and infectious mortality rate disease parameters? |
Specifically, I think I was mistakenly always assuming the transmission rate for the disease input was basically the R0 value because most of the examples I saw for the STEM documentation were in the 1 to 2 range. But, if I understand right, you are actually looking for beta, from which R0 can be calculated as beta/gamma (transmission rate/recovery rate)? I just want to verify this because if I assume an R0 of 1.3 and a recovery rate of .175 (standard flu type parameters) I would want to enter a transmission rate of .23 which is much smaller than any of the disease Beta inputs I've seen as examples.
And I understand the units for recovery rates, incubation rates etc. as per day, with their relationship to the duration of infectiousness and the incubation period being clear. But I'm having a hard time with the infectious mortality rate. I'm finding that that is typically specified as # of deaths per 1000 individuals per year. Yours is in terms of population member/time period (i.e.,day)? Do we get this from something like the case fatality ratio?
Sorry for the remedial questions but I couldn't find specific documentation on this in the Help. My main concern is that, on our end, we enter a correct *transmission rate* and not a R0 value inappropriately.
Thanks - Traci
|Re: Clarification on certain disease model parameters [message #636341 is a reply to message #635763]
||Sun, 31 October 2010 07:13
| Arik Kershenbaum
Registered: January 2010
Sorry if I've misunderstood what you're saying, but your definition of R0 seems a little mixed up.
R0 is the number of newly infected individuals that a single infected individual would generate over its infective lifetime in a wholly susceptible population- i.e. the numer of new cases that each old case replaces itself with. That is why if R0>1 then the disease spreads, otherwise it dies out.
1) R0 is a ratio; it has no units, so it is independent of the time step.
2) If the recover rate is 0.175 per day, then an individual is infectious for on average 1/0.175=5.7 days. If the transmission rate (beta) is 0.23, then over that time, they will infect 5.7*0.23 of the susceptible individuals that they come into contact with each day. R0 is dependent on the number of contacts, and cannot be defined by disease parameters alone. If they mix with 10 new (i.e. susceptible) people every day, they will infect 5.7*0.23*10=13 before recovering, i.e. R0=13 - this is a very contagious disease. But if they sneeze on just one person every other day, the R0=5.7*0.23*0.5=0.66 - the disease will not persist.
Clearly, as the number of susceptibles falls, the number of new cases falls, so R0 is only really of significance right at the start of an epidemic.
Prof Blaustein's laboratory
Department of Evolutionary and Environmental Biology
University of Haifa
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