2021 is the deadliest year in U.S. history, and new data and research provide more insight into how it got so bad.

The main reason for the increase in deaths? COVID-19, said Robert Anderson, who oversees the Centers for Disease Control and Prevention’s death tally efforts.

The agency quietly updated its provisional death toll this month. It showed 3.465 million deaths last year, about 80,000 more than the record total for 2020.

Early last year, some experts were optimistic that 2021 would not be as bad as the first year of the pandemic — in part because an effective COVID-19 vaccine is finally available.

“Unfortunately, we were wrong,” said Noreen Goldman, a researcher at Princeton University.

Experts say the COVID-19 death toll will rise to more than 415,000 in 2021, up from 351,000 the year before, as a new variant of the coronavirus emerges and an unexpectedly large number of Americans refuse to vaccinate or wear masks.

The coronavirus isn’t just the culprit. Preliminary data from the CDC also showed a slight increase in the crude death rate from cancer, and continued increases in deaths from diabetes, chronic liver disease and stroke.

Drug overdose deaths also continued to rise. The CDC doesn’t yet have a tally of overdose deaths in 2021 because it could take weeks of lab work and investigations to pinpoint them. But provisional figures through October suggest the country is on track to see at least 105,000 overdose deaths in 2021, up from 93,000 the year before.

New research released Tuesday shows a dramatic increase in overdose deaths among 14- to 18-year-olds.

For much of the past decade, the number of teenage overdose deaths has been fairly steady, at around 500 per year, according to the paper published in the Journal of the American Medical Association. They nearly doubled to 954 in 2020, and the researchers estimate the total reached nearly 1,150 last year.

The paper’s lead author, UCLA researcher Joseph Friedman, called the spike “unprecedented.”

These teenage overdose deaths account for only about 1 percent of the U.S. total. But while surveys suggest that drug use among teens has declined, the relative increase among teens has been greater than the overall population.

Experts attribute the spike to fentanyl, a highly lethal drug that has been used as heroin for years. More recently, it has also been suppressed as a prescription-like counterfeit drug that is sometimes abused by teens.

As the U.S. population grows, the total number of deaths in the U.S. typically increases year over year. However, 2020 and 2021 saw a staggering increase in the number of deaths and fatalities, largely due to the pandemic.

These national mortality trends affect life expectancy — an estimate of the average life expectancy of babies born in a given year.

With very few exceptions, life expectancy in the United States has steadily increased each year. But the CDC’s 2020 life expectancy estimate is about 77 years, more than a year and a half lower than in 2019.

The CDC has yet to report its 2021 calculations. But Goldman and some other researchers have been making their own estimates, published in papers not yet published in peer-reviewed journals.

The researchers believe that life expectancy in the U.S. will fall by another five to six months in 2021 — returning to levels seen 20 years ago.

Goldman said the reduction in life expectancy by more than two years over the past two years was “huge.”

One study looked at death data from the United States and 19 other high-income countries. The U.S. was the worst performer.

One of the study’s authors, Dr. Steven Woolf of the Commonwealth of Virginia, said: “What’s happening in the U.S. is not so much mutation as it is the level of resistance to vaccination and the public’s response to reducing the spread of the virus (such as masked and mandatory) refusal,” the university, said in a statement.

Some experts suspect that life expectancy will rebound quickly. They worry that long-term complications of COVID-19 could hasten the death of people with chronic health problems.

Preliminary and incomplete CDC data suggest at least 805,000 people died in the United States in the first three months or so of the year. This is much lower than the same period last year, but higher than the same period in 2020.

“We could end up with a slightly higher ‘new normal’ than before,” Anderson said.