What do they mean when they say we’re on our second wave of COVID-19 infections?
Understanding the waves of corona virus infection in the Philippines
In the last three months, the public has become familiar with many specialized epidemiologic terms. “Flattening the curve,” “doubling time,” and “mass testing” are just some of the phrases being bandied about. While most epidemiologists are ecstatic that the public has finally taken a keen interest in its own health (public health–no pun intended), these terms and other jargon have many nuances and are easy to misinterpret.
I remember having a Twitter conversation with someone who asked me what an epidemiologist can do to keep track of cases if we failed to do enough testing. I very thoughtlessly replied, “We count deaths.” I was promptly accused of being insensitive, when in fact mortality curves are a good measure of epidemic progression in the absence of adequate testing. Yes, it will be delayed, but excess deaths will show up in the graphs and will give one an idea of how many more deaths there are compared with historical controls.
This Twitter misunderstanding is a good example of how specialists see data in different terms than the general public. The risk of miscommunication is particularly high when academics engage mainstream media and social media. Given the extreme polarization of Philippine politics, a misstep or poorly worded reply can promptly turn into a firestorm. No better example of this is the term “mass testing,” which has been misconstrued to mean anything from testing everyone (population-based testing) to only those who are at risk (targeted massive testing).
In the end, these unfamiliar terms are easy to mess up, and can lead to a lot of needless rancor.
Most recently, there has been much debate about whether we are on our first or second “wave” of cases. From an epidemiologic perspective, a “wave” has a “peak” and so if you see multiple peaks on an epidemiologic curve, then that is a “wave.”
The current Philippine epidemiologic curve has two “peaks.” There was a tiny peak from January representing three cases from China, and there is the current wave, which peaked at 538 cases last March 31 and is still ongoing, but on the downtrend of about 200 to 300 cases per day. This needs to be seen also in the context of increasing testing capacity, which has gone from 300 tests/day to over 11,000 tests/day. So we are really capturing more cases now but it hasn’t gone beyond the 538-case peak. This means the curve was flattened. By flattened, we mean that the daily case count is steady. If it were going down, then the curve would be “falling.” Since the ECQ is not a perfect lockdown, we still expect some transmission to occur, but the growth of cases is not exponential and our healthcare system is not overwhelmed.
There was a four-week gap between the tiny “first wave” and the current “second wave.” How do we know that there wasn’t hidden community transmission that we just weren’t capturing with testing? Maybe the first and second waves are connected?
Well, we don’t know for sure, but there was active testing of PUIs at that time. Anything is possible. We do have some whole genome data that suggests that while the first “wave” was from travelers from Wuhan, the second wave came at least partially by way of India.
This does not mean Indians brought the second Covid-19 wave to the Philippines. It only means someone traveled from India and most likely got infected by a virus circulating there and brought it to the Philippines. We don’t know if the person was Indian, Filipino, or another nationality, only that he or she traveled from India to the Philippines.
If we look at the 12 whole genome sequences from the Philippines that were collected in March, these viruses all trace back to an Indian progenitor virus, which came to India from an Australian progenitor virus, which can be traced to a progenitor virus from China. The analysis is by no means complete, and we probably did have multiple introductions from the second wave but this is the evidence that we have at this time (Figures 1 and 2).
Due to the small number of available sequences, this analysis is by no means exhaustive. But it does give us an idea that the virus can come from anywhere. We cannot let our guard down and we need to limit transmission of the virus as much as possible.
Ultimately, whether this is the first or second wave doesn’t really matter in the long run. Unless we get a vaccine, there will almost certainly be another wave, and several more after that. The wave we have to deal with is the next one. Whether the peak will be higher or lower than 538 cases a day will be determined by our ability to trace, test, and treat properly. We are in a much better position to handle the next wave, with increased testing capacity, expanded isolation facilities, and intensified contact tracing. Let’s hope for a gentle ripple, not a tsunami.
Edsel Maurice T. Salvana, MD, DTM&H, FPCP, FIDSA is an internationally recognized infectious diseases specialist and molecular biologist at the University of the Philippines and the Philippine General Hospital. He is the director of the Institute of Molecular Biology and Biotechnology at the National Institutes of Health at UP Manila. He has spoken and written extensively on the Covid-19 outbreak, and serves on the Technical Advisory Group of the InterAgency Task Force for the Management of Emerging Infectious Diseases (IATF-EID).