Understanding the drastic measure that experts keep talking about
By Dr. Edsel Maurice T. Salvana, MD, DTM&H, FPCP, FIDSA
Flattening the curve. What curve? And how do you know it’s flat, bent, or whatever shape? Due to the Covid-19 pandemic, the general public is becoming familiar with specialized terms that physicians, public health experts, and scientists use in their work. RT-PCR, aerosol transmission, and reproductive number (R0 or R naught) are just some of the jargon that have spilled over to the mainstream. This spread has been aided and abetted by social media, without people necessarily understanding their proper use.
Let’s start with the curve. In this case, we are referring to the epidemiologic curve. There are two kinds of curves—a cumulative cases curve, and the daily cases curve. The shape of the curve is driven by the number of confirmed cases.
A cumulative cases curve will increase as time goes by as long as there are new cases per day. It does not matter whether the number of cases per day is getting smaller. This is because cumulative cases are additive. The only time it will be flat is when there are zero cases being added each day, when the epidemic is over. This is not the curve we refer to when we say the curve is flattening, because a flat cumulative curve heralds the end of the outbreak.
The daily cases curve is the curve you want to flatten. When cases are increasing, the daily cases curve will slope upward. Since an infectious disease spreading uncontrollably will show exponential growth, a daily cases curve that is truly terrifying will show a skyward trajectory. A case doubling time of three days will show exponential growth: If you start at 100 cases, after three days there will be another 100 new cases; after three more days it will be 200 new cases, then 400 new cases, then 800 new cases, then 1,600 new cases and so on.
Flattening the daily cases curve means keeping the number of cases steady over time. This can be through a variety of interventions such as physical distancing, eye protection, and mask wearing. One of the most important factors that determines the total number of cases overall is how early you intervene.
For instance, if you intervene at the point where there are 300 new cases every three days, flattening the curve means that you maintain the number of new cases steady at about 100 new cases a day. According to the WHO guidelines, you need to keep the daily cases steady for 14 days to say you have flattened the curve. On the other hand, if you intervene at 3,000 cases every three days, the number of cases you must maintain daily to flatten the curve is about 1,000 new cases per day for 14 days.
Both examples will result in a flattened curve, but 100 new cases per day for 14 days is a cumulative 1,400 cases, while 1,000 new cases per day in 14 days is 14,000 cases. Flattening the curve is important because it means you are controlling your outbreak. The difference in actual number of cases, however, can be quite large depending on how early you do your lockdowns or other mitigating measures.
Daily case counts are affected by backlogs. It is difficult to determine how the real curve looks until some time has passed, to give the testing delays time to catch up. Seven-day averages from time of symptom onset are used to balance out these fluctuations. Actual daily reports can be affected by testing capacity, differences in health-seeking behavior (when a patient decides to be tested), and reporting delays. The curve of the Philippines as of Sept. 9, 2020 looks like this:
After the lockdown was initiated, the curve remained relatively flat until the end of May, after which it started to increase exponentially. This increase occurred shortly after the lockdowns were eased. It also coincided with the introduction of the purportedly more infectious G614 variant into the Philippines. At the end of July, cases levelled off. This is the reason some researchers are claiming that the curve has once again flattened.
Flattening the curve only means that you are controlling the speed of transmission during an epidemic. It does not tell you whether the healthcare system is stressed out. If healthcare system capacity is limited and no improvements are made to it, then a flattened curve will still result in many deaths. If there are no more hospital beds, a flattened curve will continue to add cases and will not solve the bed shortage. If there are no available hospital beds, more people will die. The magnitude of the daily new cases is perhaps even more important than the actual shape of the curve, because above a certain number of cases a day, the healthcare system will collapse.
The pandemic is still in its early phase. There was an uproar when we spiked over 500 cases in a day in May. Five hundred cases in a day now would be a relief. We do not know just how high this can get. The early lockdown, however, enabled us to augment our capacity to take care of people. It also bought time for effective treatments to be developed, and we are now better able to ensure the survival of Covid-19 patients with severe and critical disease. Having kept our deaths low remains a singularly bright spot in our pandemic story. We have approximately 10 times fewer deaths than Spain, Italy, or France, an incredible feat considering the advanced healthcare systems of those countries.
With hope, we can continue to keep deaths low as we open our economy. Inevitably, there have been mistakes made since we are living in unprecedented times. This can still get away from us. Complacency is the enemy. Never let your guard down. We know we can control this epidemic if we follow the science, work together, and stay the course.