The Second-Biggest Wave of COVID?
Reading Time: 5 minutesWait, Was That Really the Second-Biggest Wave of COVID?, Wastewater testing data shows a peak at the beginning of January. Here’s what that really means., COVID wave: How to understand wastewater testing data.
We are, according to some commentators, coming down from the second-biggest surge of COVID-19 ever, lagging behind only the first omicron wave in early 2022 in terms of the sheer number of cases. Most countries have reported that there was a very large increase in coronavirus activity during recent weeks, even in the currently very summery Southern Hemisphere. It’s remarkable that, nearly four years into the pandemic, we are hearing about record high rates of COVID-19.
There is indeed a lot of illness going around. But the idea that this is the ‘second-biggest surge’ comes entirely from wastewater testing data. And to use that data to declare that we were recently at some kind of high point in cases is misleading.
When people are sick with COVID-19, they shed virus in their poo. If you collect samples from the sewage systems, you can then look at how much virus you are seeing in the waste, and this gives you a very rough idea of how much of the disease is around. It’s a much cruder, but drastically cheaper, way of monitoring the disease over time versus trying to test as many people as possible. It only requires public health workers to go to a handful of wastewater sites rather than collecting samples from all over the country. If you look at the dashboard for the Centers for Disease Control and Prevention, you can see that the wastewater viral activity levels from December 2023 to January 2024 were actually slightly above those seen during the December 2022 to January 2023 surge.
The problem is that you can’t really use wastewater data to make these kinds of comparisons; this has tons of limitations. The number of sites that the CDC gets data from is not static, with some places reporting data much less frequently than others. In August 2023, there were nearly 1,400 sites reporting data to the CDC, up from about half of that earlier in 2023. By January 2024, that number had dropped back down, to more like 800. Some states, like California and Ohio, have regular, recent data, while other areas of the U.S., like Oklahoma, Arkansas, and Oregon, haven’t reported complete data, or any data, in at least three months. It all just fluctuates.
Wastewater surveillance also doesn’t cover the entire country. It’s quite good for more-urban areas, but there are far fewer samples from more-remote places. About 20 percent of U.S. households are on septic systems, which are not captured by wastewater surveillance at all. Using even perfect and complete wastewater data to infer how many cases there are is complex. We know that higher levels of disease in poo are linked to more viral activity, but putting a number on that requires quite a few assumptions. Some variants may cause people to poo out more or less viral RNA, for example. Some variants have caused longer or shorter acute disease periods. Local vaccination rates almost certainly make a difference in how much virus you see in wastewater.
Wastewater surveillance is a useful as a general signal; it gives public health experts a clue as to what is happening in the community, and whether they should watch out for an uptick in hospitalizations and deaths. It’s far from a firm readout of case numbers. We can’t really use it to say whether one surge is bigger than another. It’s possible the current uptick is driven in part by factors other than the current number of actual cases, which is a figure that is very hard to know. In addition to all this, most wastewater sites in the U.S. did not start reporting data until December 2021, which means that we can only compare viral activity since the end of 2021. It’s possible that prior surges, as measured by wastewater data, were bigger.
It has always been tricky to nail down the number of cases, even when testing individuals was more common. Hospitalizations and deaths are, in some respects, a more reliable way to get a handle on the current status of the disease. This is because people who get very sick for any reason are much more likely to be tested for COVID-19 than the average person is, and even if this metric scoops up some people with mild illness, comparing COVID hospitalizations at one time with that of another time is an apples-to-apples comparison. Death counts are robust because it’s quite rare that a death caused by the disease is not recorded as such.
If you look at both hospitalizations and age-adjusted deaths for the U.S., you can see that the current wave is also one of the smallest of the entire pandemic, and perhaps even the smallest. The data is a bit lagged—as of writing, the CDC has updated through Jan. 6. It is possible that the wave will be a bit higher than the winter 2022–23 peak, but if it is, it’s unlikely to be by much. (You see the same thing echoed in other countries’ data. In the U.K., where the testing data is more robust, it looks as if the most recent wave has caused the fewest hospitalizations per case of any surge so far in the pandemic.)
Even if we could prove that we are in the second-highest surge, casewise, right now, that doesn’t tell us much about where we are in the pandemic. It’s a fundamental point about public health: At the end of the day, we don’t really care about number of infections caused by a disease, per se. The reason we care about someone testing positive for a disease is because diseases cause ill health, like hospitalization and death. In 2020 a large proportion of those who tested positive for COVID-19 were very sick, so we cared deeply about the number of cases. As the disease has decreased substantially in severity—including causing far fewer cases of long COVID than in previous years—simple case estimates become less and less useful.
COVID-19 still causes a large amount of ill health. Even at much lower severity, it’s still a big public health problem. But in terms of where we are in the pandemic now, as compared to earlier, we are somewhere much better.
It’s true that, by this single measure, we have recently had the second-biggest surge since the United States started collecting countrywide wastewater data, but it’s also true that by many of the more meaningful measures, the recent wave has so far been the smallest.
Perhaps the most useful thing to say about all this data is that even if there were more cases recently, the large number of people getting sick hasn’t caused the same huge burden of severe disease and death that we saw during the initial two years of the pandemic. It’s possible that there were huge numbers of people catching COVID-19, but we know that most of them didn’t get nearly as sick, by any indicator we have available, as people were getting at the start of the pandemic. Due to the immunity we’ve built up since COVID-19 emerged—and our fantastic vaccines—we are no longer at the same risk we were as when we first started catching the illness.
Reference: https://slate.com/technology/2024/01/covid-wave-second-biggest-maybe-not-wastewater.html
Ref: slate
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