An interesting story today at the Atlantic based on a pre-print (not yet peer-reviewed) study of hospital patients with COVID. The story points out that hospitalizations have been considered a reliable metric for tracking the rise and fall of the disease over time.

On the one hand, death counts offer finality, but they’re a lagging signal and don’t account for people who suffered from significant illness but survived. Case counts, on the other hand, depend on which and how many people happen to get tested. Presumably, hospitalization numbers provide a more stable and reliable gauge of the pandemic’s true toll, in terms of severe disease.

The problem with using hospitalizations as a metric is that you don’t really know how sick the patients were. For instance, was the patient intubated or put on oxygen or were they only there for observation. In some cases, they might even have been there for something else and only found out they had mild COVID after being tested upon admittance.

The authors of this new study decided to try to find out what percentage of the hospitalizations were serious cases. The did so by looking at 50,000 COVID-19 patients in VA hospitals across the country. Any patient who required oxygen or had blood oxygen levels below 94 percent was considered a serious case. Any patient who didn’t meet those criteria was considered a mild case.

The study found that from March 2020 through early January 2021—before vaccination was widespread, and before the Delta variant had arrived—the proportion of patients with mild or asymptomatic disease was 36 percent. From mid-January through the end of June 2021, however, that number rose to 48 percent. In other words, the study suggests that roughly half of all the hospitalized patients showing up on COVID-data dashboards in 2021 may have been admitted for another reason entirely, or had only a mild presentation of disease.

This increase was even bigger for vaccinated hospital patients, of whom 57 percent had mild or asymptomatic disease. But unvaccinated patients have also been showing up with less severe symptoms, on average, than earlier in the pandemic: The study found that 45 percent of their cases were mild or asymptomatic since January 21. According to Shira Doron, an infectious-disease physician and hospital epidemiologist at Tufts Medical Center, in Boston, and one of the study’s co-authors, the latter finding may be explained by the fact that unvaccinated patients in the vaccine era tend to be a younger cohort who are less vulnerable to COVID and may be more likely to have been infected in the past.

There are some limitation to this study including that the VA tests every incoming patient for COVID but not every hospital does this. Also the study ended in June or right about as the delta wave was really taking off. So it’s possible the situation has changed a bit recently.

Still, it appears a COVID hospitalization in 2021 wasn’t as likely to be serious as one in 2020. So using it as if it were a consistent metric across the entire pandemic could be pretty misleading. I think most people who hear about COVID hospitalizations are still thinking back to the early days of the pandemic and images of wards full of patients on ventilators. That is still happening but if it only represents about half of current hospitalization numbers, we ought to let people know that.

The solution of course is to look more closely at the data. This study will need to be peer-reviewed and then additional studies will need to look at non-VA hospitals to see if the same is true outside the VA system. The goal is to provide a more accurate picture of what is happening. Dr. Shira Doron, an epidemiologist from Tufts, told the Atlantic, “we should refine the definition of hospitalization. Those patients who are there with rather than from COVID don’t belong in the metric.”