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Boston researchers find COVID hospital count makes vaccines seem less effective

Staff worked at a COVID-19 ward at Brooklyn Hospital Center in New York on March 23, 2020.VICTOR J. BLUE/NYT

Widely used methods for counting US COVID-19 hospitalizations can make vaccines appear less effective than they actually are, according to a group of Boston-based researchers.

The researchers say their work raises questions about how the US should approach future booster-shot campaigns and how the country measures the severity of the pandemic. It may also have implications for billions of dollars in bonus payments that have gone to hospitals caring for COVID patients covered under Medicare.

The researchers looked at a group of more than 8,000 patients who had been admitted to Department of Veterans Affairs hospitals since September 2021 and who had a positive COVID test – the same criteria that most states and the US Centers for Disease Control and Prevention use to tally daily COVID hospitalizations.


They found that about 40 percent of COVID-positive patients were likely in the hospital primarily for something other than the virus. About 60 percent of patients met the criteria for moderate or severe disease, meaning they had low blood-oxygen levels or needed supplemental oxygen. The share of patients getting COVID drugs – a sign that doctors were treating them for more severe COVID, not another condition, was substantially lower, and particularly so among those who had been vaccinated or boosted.

“Many of the hospitalizations where there’s a positive test were not necessarily initiated because there’s a Covid infection,” said Westyn Branch-Elliman, an infectious diseases specialist at VA Boston Healthcare System and an associate professor at Harvard Medical School. She is one of the co-authors of the research paper, which has been accepted for publication in the medical journal Infection Control & Hospital Epidemiology.

Widely used hospitalization data published by many states as well as the Centers for Disease Control and Prevention counts any patient with a positive COVIS-19 test as a COVID hospitalization. But as vaccines have become widespread, some hospitals and research groups argue that measure is too blunt, and that it inadvertently captures people who are hospitalized for other causes but also happen to have a mild case of the infection.


Other experts, including some at the CDC, say it’s more appropriate to capture a wider group of patients in hospitalization counts, since Covid can exacerbate other underlying conditions that can lead to a hospitalization.

The rollout of vaccines “made it critical that we characterize ‘with COVID’ versus ‘for COVID’ hospitalizations so that we don’t undermine confidence in vaccines by making them look less effective than they are,” said Shira Doron, an infectious disease doctor and hospital epidemiologist at Tufts Medical Center, another of the authors of the paper. The work by the researchers, who are at Tufts, Harvard University, and the Department of Veterans Affairs, has been paralleled by other groups, all pushing for better ways to understand the virus’s impact.

A handful of states have started breaking out incidental or “with” COVID hospitalizations from cases that are caused by the virus. Among states publishing those figures, the methods vary, as do the numbers. In New York, Massachusetts and New Hampshire during the first week of May, the share of people in the hospital with a positive COVID test who were being treated primarily for the illness ranged from 28 percent to 49 percent.

Because COVID-19 is such a complicated disease, affecting many different parts of the body, none of those metrics are perfect. The CDC, through its Covid-NET hospital surveillance network that covers about 10 percent of the US population, also analyzes “with” versus “for” COVID hospitalizations. The Atlanta-based agency uses a methodology that sweeps up more patients with mild symptoms but whose infections may have exacerbated their diabetes or a cardiovascular condition, for example, leading to a hospital admission.


Read more: The Covid metrics we need for the next phase of the pandemic

“The ‘admitted for’ COVID subject is very hairy,” said Jeffrey Klann, a Harvard University researcher who has conducted a separate analysis of hospitalizations and found similar trends to the Boston team. “There’s definitely some gray area in the tough cases,” he said.

Studies of how effective vaccines are over time, particularly those that have looked at how protection might be fading over time, have relied on less precise methods. One piece of research published by the CDC found that the effectiveness of a third dose of vaccine against being hospitalized fell from 91 to 78 percent in just two months. Those signs of declining efficacy have led to discussion about the need for additional booster shots.

But the CDC study counted hospitalizations as anyone with even mild symptoms of the disease – such as a fever, a cough, diarrhea, or vomiting – as well as a positive test. So, too, did another study on vaccine efficacy that the agency cites on its vaccine efficacy website.

“Hospitalized patients often have some of the symptoms listed for reasons other than COVID,” Doron said. “More importantly though, the patient can be symptomatic from COVID and not in the hospital due to COVID.”


Including those patients in an efficacy study can cloud whether or not the hospitalization is from COVID or how severe the case is, Branch-Elliman said. A better understanding of hospitalizations is “very important for making decisions about boosting campaigns,” Branch-Elliman said, “and also provide some reassurance to vaccinated people that they are protected.”

It’s been a politically fraught topic, as well. Close to 1 million people in the US have died of COVID-19, and the illness still results in almost 300 deaths a day. Early in the pandemic before vaccines, some who doubted the severity of the disease claimed that hospitalization numbers were inflated. Researchers who study those trends say that’s not the case, and that the rise of incidental hospitalizations is a more recent trend.

There are also financial implications for the nation’s Medicare program, which provides health insurance for more than 60 million people ages 65 and over. Since the beginning of the pandemic, the program has paid hospitals a 20 percent bonus for admissions with a positive COVID-19 test. That also applies to private health insurers who participate in the Medicare Advantage program.

From the start of the pandemic through Nov. 20, 2021, more than 1.6 million people covered by Medicare have been hospitalized. A typical hospitalization for COVID costs Medicare about $24,000, according to the Centers for Medicare and Medicaid Services, the federal agency that oversees the program.


That’s led at least one insurer in Medicare Advantage to start looking more closely at hospitalizations. Humana Inc., while reporting first-quarter financial results this month, said that 25 to 30 percent of the COVID-19 hospital admissions it paid for during the Omicron wave were “with” COVID, not “for” COVID.

Humana, which covers about 5.1 million people through its Medicare plans, used a conservative method for counting admissions for COVID, treating any patient with a known major symptom of the virus as a “for” COVID admission, according to a person familiar with the company’s review. It was part of an effort to get a better understanding of how what it was hearing anecdotally was playing out in data, and what the financial implications were, said the person.

Much of the data US health officials compile comes first from state and local health departments, or directly from health care providers. That’s made nationwide standards for data more challenging to implement, and health officials have sometimes been reluctant to add additional burdens to hospitals already dealing with large volumes of patients and other reporting requirements.

“There’s a lot of hesitancy to do one more thing,” Doron said. “Everybody knows we’re already overwhelmed.”