The nature of growth is geometric
CLINICAL MATTERS
Dr. Edsel Maurice T. Salvana
It starts with one.
The COVID-19 pandemic that has infected over 140 million and killed more than three million people started with one person who was infected by a still unknown animal. Coronaviruses usually originate in bats, which can then infect humans directly, or through another animal. SARS was transmitted from bats to bamboo rats or civet cats, while MERS was transmitted from bats to camels. From one infected person, thousands then millions of people can be infected if the transmission chain is maintained.
The nature of infectious diseases is that growth is geometric. Instead of going from one to two to three to four to five patients, the infection goes from one to two to four to eight to sixteen patients and so on. The most recent surge in Metro Manila caught everyone by surprise because it grew so fast. Growth of infectious diseases can be truly explosive.
There are two main measurable components that drive a pandemic. The R0 (R naught) is the number of people that get infected from one case. The case doubling time (CDT) is the amount of time it takes for the number of infections to double. For a disease like COVID-19 with an average incubation period of about five days and an R0 of 2, the number of cases is expected to double every five days if no mitigating measures are instituted.
For example, an R0 of 2 means that if there are initially one hundred people with infection, there will be two hundred more people infected in five days, four hundred more people in another five days, another eight hundred people in another five days, and so on. If measures that decrease transmission are initiated, the CDT can be brought down. The R0 decreases and is then referred to as the time-varying reproduction number, or Rt.
When dealing with infectious diseases, one cannot think in a linear fashion. One must think in exponential terms and expect that an epidemic can accelerate rapidly out of control. Time is of the essence and decisions need to be made quickly. Each day counts and vacillation can literally cost thousands of lives. Mexico, which locked down approximately 10 days after us, ended up with more than 13 times our number of deaths and double the number of cases. A difference of 10 days resulted in nearly 200,000 preventable deaths.
We have had three surges in the Philippines. The first surge occurred last March 2020 coincident with the discovery of community transmission in the Philippines. This surge peaked at 538 cases. This surge was terminated with the most stringent and extensive lockdown ever imposed. At one point the entire island of Luzon was under enhanced community quarantine to try to control the spread of infection. It worked. The lockdown saved tens of thousands of Filipinos, but it also came at great economic cost. The lockdown was finally eased in June 2020.
Following the downgrade of community quarantine, cases predictably started to climb again. This coincided with the introduction of the B.1 lineage from returning overseas Filipinos. Lineage B.1 carries the D614G mutation, which makes it three to nine times more transmissible than the other lineages. The second surge accelerated quickly and peaked at 6,871 cases. It also resulted in a two-week MECQ after hospitals became dangerously full before cases started going down.
Cases continued to ease and stabilized at around 1,000 cases a day throughout December 2020 and January 2021. The concern of a post-holiday surge and the introduction of new variants prompted travel bans and strict quarantine procedures on travel from countries with reported variants of concern. Cases remained manageable through February 2021, which persuaded the IATF into further loosening capacity controls.
Cases began to rise anew in early March 2021 and rapidly accelerated despite localized lockdowns, curfews, and a stricter GCQ. Hospitals in NCR rapidly filled up and had to turn away patients. Cases peaked at 15,280 new COVID-19 infections in one day. Deaths started to increase as people died in emergency rooms while waiting for admission. ECQ was declared for two weeks. Cases have now thankfully started to come down and have continued to decrease even as the lockdown was downgraded to MECQ.
The most recent Rt calculated by the Department of Health is close to 1.002, backdated to March 31, 2021. This indicates that even prior to the ECQ, the localized lockdowns, curfews, and the NCR plus bubble had succeeded in decreasing the Rt substantially. Rt should not be calculated from cases less than two weeks old because daily cases are aggregates of patients with different dates of symptom onset. These need to be spread out according to when symptoms started to capture an accurate picture of when actual infections occurred. Most Rt values reported by DOH are qualified as two to three weeks old. Nevertheless, even an Rt close to 1.0 is not enough if the healthcare capacity is fully utilized. Incident cases will continue to add patients who need to be admitted to the hospital and will have nowhere to go.
Are hard lockdowns the only way out of a surge? When cases are out of control and the healthcare system is overwhelmed, it is the only way to rapidly get new infections down. An article that came out early in the pandemic was “The Hammer and the Dance” by Tomas Pueyo. In it, the author outlined a strategy of an initial hard lockdown (the hammer) followed by mitigating measures as the economy opens to keep the virus under control (the dance). Occasionally, the virus still gets out of control necessitating short periods of lockdown. Lockdowns, due to their great cost, should only be used as a last resort when the healthcare system is in danger of being overwhelmed.
With worldwide cases surging higher than ever and more transmissible variants taking over, both short-term and long-term solutions are required for our nation to survive. In order to quickly reopen the economy, healthcare capacity must be increased to account for the expected bump in cases as a result of more people moving about. Since adding healthcare capacity is resource-intensive and takes time, closer adherence to public health standards to decrease transmission is imperative while the supply of hospital beds is being increased. The long-term solution is mass vaccination, which is currently beset with supply issues and problems with vaccine hesitancy. In these challenging times, the entire nation needs to be united. We need to help each other as the virus makes its last stand. How many of us survive to see the end of the pandemic truly depends on our ability to protect one another.