The average American life expectancy ticked downward for the second straight year in 2016, on the back of surging drug overdose deaths, according to data released Thursday by the National Center for Health Statistics at the U.S. Centers for Disease Control and Prevention. And while the nation hasnt experienced a back-to-back drop in life expectancy since the 1960s, the CDC says the opioid crisis is shaping up to extend this decline for a third consecutive year, a milestone that hasnt been seen since the Spanish flu pandemic in 1918.

U.S. life expectancy fell to an average of 78.6 years in 2016, dropping 0.1 for the second year in a row, according to the CDC report . The slide was driven by higher death rates among young and middle-aged Americans, with those aged 25-34 experiencing the largest increase. The death rate among Americans aged 65 and older actually inched downward between 2015 and 2016.

The overall decline in longevity came as drug overdose deaths exploded in 2016 to a total of 63,600, around 42,000 of which involved opioids, according to CDC data . Although these numbers have been rising steadily since 1999, the 21 percent jump in deaths over 2015 was the largest annual increase so far. Drug overdose deaths involving synthetic opioids like fentanyl and fentanyl analogs have increased an average of 88 percent each year from 2013 to 2016, helping to drive the surge.

The drug overdose deaths seem to be driving what were seeing, said Bob Anderson, chief of the mortality statistics branch at the CDCs National Center for Health Statistics. Its certainly a key factor in the decline over the past two years.

The United States hasnt seen two straight years of falling life expectancy since 1962 and 1963, amid a deadly outbreak of influenza. And the one-year decline in life expectancy measured last year marked a first since 1993, at the height of the AIDS epidemic.

Although the U.S. has grappled with spikes in drug deaths before, the current overdose epidemic is also taking place as the rate of reduction in heart disease deaths begins to slow.

In the past, those drug mortality increases have been more than completely offset by declines in cardiovascular disease mortality heart disease and stroke mostly, said Anderson. But now those declines have slowed down and kind of flattened out. And as a result, these increases in drug overdose mortality are dominating the picture for overall mortality.

As concerned as he is by the 2015 and 2016 data, Anderson says it looks like it could be the beginning of a longer trend.

What were seeing in our surveillance system for 2017 so far doesnt look good, doesnt look any better than 2016, he said. Assuming that things kind of stay the way they are with other causes of death, and the drug overdose mortality goes up, I think were in for more of the same and maybe a third year in a row of declining life expectancy, which we havent seen for 100 years, since the Spanish flu in 1916, 1917 and 1918.

It would be a grim marker for the U.S., matching a period in which more than one-quarter of the U.S. population reportedly got infected by the disease, leaving some 675,000 Americans dead. Although the current trajectory is troubling, the scale is quite different, as the Spanish flu ultimately sent the average American life expectancy plummeting a dozen years, to just 39 years, in 1918.

Thursdays report could inject new urgency into President Donald Trumps plan to address the opioid epidemic. Trump formally declared a public health emergency for the crisis in October, though he has faced criticism for not tackling the problem more aggressively.

The public health emergency fund currently holds only around $57,000 , according to the Department of Health and Human Services, and Trump has not allocated the level of funding experts say is necessary to truly address the problem. Last month, the White House placed the true cost of the opioid epidemic at over $500 billion in 2015 .

We really do need to get a handle on this, and I hope that this data will be of use to policymakers in formulating something that will take care of this problem. These are deaths that should be entirely preventable, said Anderson. The message I want to send is that this is problem for our nation and we need to do something about it.