I said many reasons (formerly) at Why should Ho Chi Minh City stop the blockade? Reading through the daily data in Ho Chi Minh City today, I have one more reason to stop the lockdown: the contagion coefficient in HCM has been decreasing.
Perhaps there is no need to elaborate on what ‘reproduction ratio’ (or R) means because I explained it last year when the epidemic first started. Roughly, if 1 person gets infected and spreads it to 2 other people, then the coefficient of contagion CHEAP = 2. For this pandemic, WHO based on Wuhan data estimates that R ranges between 1.4 and 2.5.
The change of the coefficient of contagion is very important, because it is one of the factors to decide the policy. If CHEAP > 1, the epidemic is still contagious; if CHEAP < 1 then the epidemic is declining and will 'decline' over time.
Last year, using data from Wuhan and the number of daily cases in Vietnam, I estimated that CHEAP = 1.08, but the 95% probability can range from 0.87 to 1.33. Then the coefficient CHEAP dropped below 1 and indeed, the epidemic was under control.
What about this year with HCMC?
I use data on daily infections (since 1/7/21) and calculate R again for each week. The chart below shows that at the beginning of July, the coefficient CHEAP = 1.33 (95% confidence interval: quartile 1.12 to 1.61). But then it fluctuates over time, and most importantly, from the beginning of September/21 CHEAP decreased to less than 1. From the week of September 13, R = 0.94 (95% confidence interval: 0.89 to 0.99).
Another metric that is also important is the ‘actual spread’ (Re or effective .). CHEAP). It can be estimated by the formula:
Re = R*(1 – P),
Inside P is the percentage of the population that has been immunized through a vaccine or has recovered from infection. We know that in HCMC the number of people receiving one dose is 90%, and with CHEAP = 0.9 then Re = 0.09. Even if the 2 dose ratio is 30% Then Re = 0.63.
In summary, the above epidemiological explanation shows that the epidemic in HCMC is declining and the actual transmission coefficient is low or very low. This means that HCM should stop the blockade. Stop as soon as possible.