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R0 represents the average number of susceptible individuals an infected person will transmit the disease to.
R0 = Attack rate x Contacts
Attack rate (the percent chance that a contact will get the disease).
If the R0 is < 1, a disease outbreak should wane over time, and if its >1, cases would continue to increase.
Seasonal flu has an R0 of around 1.5. The Spanish influenza of 1918-1919 had an R0 of 2. Chickenpox, a fairly infectious condition, has an R0 of around 5.
To predict the severity of a new disease, we need to look at both the basic reproduction number and the case fatality rate.
The case fatality rate of Spanish flu was reported to be as high as 10%.
Before treatment was available, HIV, with an R0 of around 6 globally, had a near 100% mortality rate. Smallpox, with an R0 of 5, had a mortality rate of 30% in the unvaccinated. Bubonic plague: R0 of 3, untreated mortality rate of 60%.
COVID-19 has R0 of 2.5 and a reported case fatality rate of around 2%.
Case fatality rate is defined as the number of fatal cases divided by the number of total cases.
In COVID-19, fatal cases have probably been assessed accurately; people who are that sick generally end up in hospitals. However, the number of total cases are probably being missed by huge margins, perhaps even an order of magnitude, as asymptomatic and mildly symptomatic people may not be getting tested. If this is the case, the case fatality rate would decrease as screening improves.
How to change the R0 number
One can do that by addressing the two elements inside it: the number of contacts an infected person has and the attack rate of the disease.
Limiting potential contacts can be achieved by means of isolation and quarantine.
Attack rate can reduce with the use of masks, handwashing, and vaccination, as and when a vaccine becomes available.
This depends on identifying cases, and it is still unclear as to whether transmission can occur in the asymptomatic period. The latent period is 5 days.
Dr KK Aggarwal
President CMAAO, HCFI and Past National President IMA