The year 2020 has been abnormal for mortalities. At least 356,000 more people in the United States have died than usual since the coronavirus pandemic took hold in the country in the spring. But not all of these deaths have been directly linked to Covid-19.

More than a quarter of deaths above normal have been from other causes, including diabetes, Alzheimer’s disease, high blood pressure and pneumonia, according to a New York Times analysis of estimates from the Centers for Disease Control and Prevention.

Some of these additional deaths may actually have been due to Covid-19, but they could have been undiagnosed or misattributed to other causes.

Many of them are most likely indirectly related to the virus and caused by disruptions from the pandemic, including strains on health care systems, inadequate access to supplies like ventilators or people avoiding hospitals for fear of exposure to the coronavirus.

40,000 extra deaths from diabetes, Alzheimer’s, high blood pressure and pneumonia

Research has shown that people with underlying health conditions such as diabetes, high blood pressure and heart disease are particularly vulnerable to severe illness and death if they contract Covid-19.

In several states, deaths attributed to diabetes are at least 20 percent above normal this year.

Prolonged economic stress on families during the pandemic could also be contributing to increased deaths among those with chronic illnesses.

“You end up having to choose between your prescription medications or buying groceries or keeping a roof over your head,” said Steven Woolf, director emeritus of the Center on Society and Health at Virginia Commonwealth University, whose research has also shown deaths from other causes to be higher than normal.

At least 10 states have seen deaths from high blood pressure — a common comorbidity like diabetes — rise even higher than the national percentage. These may include deaths from heart failure, kidney failure or stroke.

Many people who die from high blood pressure are also at high risk for severe Covid-19, so some of these deaths could be Covid-19 deaths that are missed, according to Robert Anderson, chief of the mortality statistics branch at the C.D.C.’s National Center for Health Statistics.

Nationwide, deaths from Alzheimer’s disease, which usually affects older adults, are 12 percent above normal this year, with several Southern states seeing larger increases. This could be related to challenges in providing adequate care in nursing homes during the pandemic — deaths in nursing homes account for more than a third of the nation’s total coronavirus toll. The virus may have also aggravated some of these patients’ existing health conditions.

Other factors related to the pandemic like social isolation and challenges in getting emergency services could also have contributed to deaths, Dr. Woolf said.

Many of the higher than normal deaths from pneumonia are most likely Covid-19 deaths that were not identified as such, especially earlier in the pandemic when coronavirus tests were scarce. Chest X-rays from the virus and pneumonia also look especially similar, experts said.

New York City, an early epicenter of the pandemic, has seen pneumonia deaths reach about 50 percent above normal, more than double the percentage in the states with the highest rates.

As the pandemic has progressed, coroners and medical examiners have become better at recognizing the deaths caused by the virus.

Counting deaths takes time, and many states are weeks or months behind in their reporting. These estimates from the C.D.C. are adjusted based on how mortality data has lagged in previous years.

Dr. Woolf also warned that many people who are not captured in mortality statistics may still have adverse health outcomes.

“A person who survived the pandemic may end up deteriorating over the next few years because of problems that happened during the pandemic,” he said. This could include those who have missed routine checkups or have had delays in receiving proper treatment for an ailment.