THERE has been a lot of concerns lately when the enhanced community quarantine (ECQ) will be over. It will be on the 15th of May for Davao City and Metro Manila. I decided to repost this letter addressed to policy makers, especially from Dr. Robert Dennis Garcia, an Infectious Medicine specialist currently practicing in Makati Medical Center.
Dear friends and policy makers,
When May 1 comes and we go out of our homes, we will not be immune to Covid-19 any more than we were before March 16, except for the few of us who were infected in the preceding six weeks. Indeed, if 10,000 Filipinos would have been infected by May 1, this would only amount to 0.009 percent of the population, hardly a dent towards obtaining herd immunity against the dreaded virus.
At the onset of the lockdown on March 16, the doubling time of Covid-19 cases was five days. By April 15, as an effect of the lockdown, the doubling time had prolonged to 13.8 days.
The Philippine population is 110 million. To achieve a sizable herd immunity, we generally have to have at least 70 percent of the population to have been infected by Covid-19, and to have recovered.
Over how long a time can we achieve that 70 percent?
Once the lockdown is lifted on May 1, if there are 10,000 Filipinos infected by then, and if people behave as they did before March 16 (inadequate social distancing, unimpeded outside activities, no substantial use of masks in public), it will require roughly thirteen doubling times, if viral spread is transferred efficiently from person to person, in order to infect 82 million Filipinos. With a five-day doubling time, the date by which 82M Filipinos would have been infected will be as early as July 5. Eighty-two million is 74 percent of the Philippine population, enough for herd immunity.
As the viral transfer becomes inefficient when more people become infected and recover, if the doubling time is spaced out to 9 days, the new date by which 82M Filipinos will be infected will be August 26.
Currently, on April 15, due to the lockdown’s inhibiting effect on the spread of the virus, the doubling time has prolonged to 14 days. If we assume that when the lockdown is lifted on May 1, and we can still have a 14-day doubling time (because people will be good at preventing horizontal transfer as they go back to work and school), we will achieve the 82M Filipinos infected by October 17 (i.e., thirteen doubling times).
We’re out of space. Let’s continue this by next issue. Until then.
Dr. Fritz