2 kids are arguing: one claims the other is a Geek.
"R too."
"R naught"
Which one is destined to become an epidemiologist?
Yesterday the CDC published a new analysis of the natural R0 (R naught) - the base reproduction rate.
Initial analysis going back to early January estimated it as being roughly 2.5 - i.e. every infected person then passed the virus on to 2.5 additional people.
The components of reproduction rate include the likelihood of passing the infection from one person to another in any individual contact, the average number of social contacts per day, and the number of days that a person remains infectious. So, if a person continues to shed virus for 5 days, is pretty gregarious and meets 30 persons per day, and is likely to transfer virus in 1 our of 30 contacts, R0 becomes 5.
The CDC went back and looked again at raw data from Wuhan, and correlated it with phone records and travel (train and flight) records and then at how fast the illness blossomed in the destinations. The methodology was well beyond my understanding, but they came up with a median R0 estimate of 5.6.
Well, isn't that water under the bridge? We've already passed that and it's just old history.
Not so fast: There is a relationship between R0 and the level of herd immunity required to stop the spread of disease. For instance, if R0=4 and there is no immunity, 4 people will be infected by one sick person. But if 3/4 of the population has been vaccinated, that same sick person will expose 4 additional people, but only one will become infected and the number of infected in the population will remain stable. Once herd immunity rises to 0.76, the epidemic will gradually die out.
This is expressed by 1-(1/R0) as the herd immunity threshold.
So, if R0 is 2.5, then 1-(1/2.5) is 1-0.4 = .6 or 60%. 60% of the population needs to become immune or dead for the epidemic to stop.
If we, through whatever contact limitations are adopted, drop the R0 to 2.0, we reach the threshold at 50% immunity. But, if the CDC recalculation of R0 is correct, we don't reach the threshold to stop the advancing epidemic until 82% of the population has been immunized by illness or vaccine. So, the new CDC analysis raises the bar substantially.
Of course, R0 is going to be a moving target: even if it started at 5.6 before we knew what was happening, it has fallen progressively to 2.5 and to 2.0 and down below 2.0. After the severe lockdown in Wuhan, the estimated R0 fell to about 0.3. That means that each person, on average, infected less than one additional person and eventually transmission of the virus within the population ceased.
So where we are at in the US (and Frasier) is to reduce R0 below one by modifying the only term in the equation that we can modify: number of contacts per day. Once we stop transmission, we have to be wary of loosening restrictions because if we return to eat, drink, and shake hands, then R0 rises again to near 5 and we'll continuing the yo-yo pattern (llosening and tightening social distaning rules) until we have infected about 80% of the population, or until we develop a vaccine and have immunized (by a combination of infections and vaccinations) about 80% of the population.
Support your local vaccine developers!
"R too."
"R naught"
Which one is destined to become an epidemiologist?
Yesterday the CDC published a new analysis of the natural R0 (R naught) - the base reproduction rate.
Initial analysis going back to early January estimated it as being roughly 2.5 - i.e. every infected person then passed the virus on to 2.5 additional people.
The components of reproduction rate include the likelihood of passing the infection from one person to another in any individual contact, the average number of social contacts per day, and the number of days that a person remains infectious. So, if a person continues to shed virus for 5 days, is pretty gregarious and meets 30 persons per day, and is likely to transfer virus in 1 our of 30 contacts, R0 becomes 5.
The CDC went back and looked again at raw data from Wuhan, and correlated it with phone records and travel (train and flight) records and then at how fast the illness blossomed in the destinations. The methodology was well beyond my understanding, but they came up with a median R0 estimate of 5.6.
Well, isn't that water under the bridge? We've already passed that and it's just old history.
Not so fast: There is a relationship between R0 and the level of herd immunity required to stop the spread of disease. For instance, if R0=4 and there is no immunity, 4 people will be infected by one sick person. But if 3/4 of the population has been vaccinated, that same sick person will expose 4 additional people, but only one will become infected and the number of infected in the population will remain stable. Once herd immunity rises to 0.76, the epidemic will gradually die out.
This is expressed by 1-(1/R0) as the herd immunity threshold.
So, if R0 is 2.5, then 1-(1/2.5) is 1-0.4 = .6 or 60%. 60% of the population needs to become immune or dead for the epidemic to stop.
If we, through whatever contact limitations are adopted, drop the R0 to 2.0, we reach the threshold at 50% immunity. But, if the CDC recalculation of R0 is correct, we don't reach the threshold to stop the advancing epidemic until 82% of the population has been immunized by illness or vaccine. So, the new CDC analysis raises the bar substantially.
Of course, R0 is going to be a moving target: even if it started at 5.6 before we knew what was happening, it has fallen progressively to 2.5 and to 2.0 and down below 2.0. After the severe lockdown in Wuhan, the estimated R0 fell to about 0.3. That means that each person, on average, infected less than one additional person and eventually transmission of the virus within the population ceased.
So where we are at in the US (and Frasier) is to reduce R0 below one by modifying the only term in the equation that we can modify: number of contacts per day. Once we stop transmission, we have to be wary of loosening restrictions because if we return to eat, drink, and shake hands, then R0 rises again to near 5 and we'll continuing the yo-yo pattern (llosening and tightening social distaning rules) until we have infected about 80% of the population, or until we develop a vaccine and have immunized (by a combination of infections and vaccinations) about 80% of the population.
Support your local vaccine developers!
