New U.S. research on long COVID-19 provides fresh evidence that it can happen even after breakthrough infections in vaccinated people, and that older adults face higher risks for the long-term effects.

In a study of veterans published Wednesday, about one-third who had breakthrough infections showed signs of long COVID.

A separate report from the Centers for Disease Control and Prevention found that up to a year after an initial coronavirus infection, 1 in 4 adults aged 65 and older had at least one potential long COVID health problem, compared with 1 in 5 younger adults.

Long COVID refers to any of more than two dozens symptoms that linger, recur or first appear at least one month after a coronavirus infection. These can affect all parts of the body and may include fatigue, shortness of breath, brain fog and blood clots.

Coronavirus vaccines that help prevent initial infections and serious illnesses provide some protection against long COVID but mounting research shows not as much as scientists had first hoped.

The veterans study published in Nature Medicine reviewed medical records of mostly white male veterans, aged 60, on average. Of the 13 million veterans, almost 3 million had been vaccinated last year, through October.

About 1%, or nearly 34,000, developed breakthrough infections. Lead author Dr. Ziyad Al-Aly noted that the study was done before the highly contagious omicron variant appeared at the end of the year and said the rate of breakthrough infections has likely increased.

Breakthrough infections and long COVID symptoms were more common among those who had received Johnson & Johnson’s single-dose shot compared with two doses of either Moderna or Pfizer vaccines. Whether any had received booster shots is not known; the first booster wasn’t OK’d in the U.S. until late September.

Overall, 32% had long COVID symptoms up to six months after breakthrough infections. That’s compared with 36% of unvaccinated veterans who had been infected and developed long COVID.

Vaccination reduced the chances for any long COVID symptoms by a “modest” 15%,” although it cut the risk in half for lingering respiratory or clotting problems, said Al-Aly, a researcher with Washington University and the Veterans Affairs health system in St. Louis. These symptoms included persistent shortness of breath or cough and blood clots in lungs or veins in the legs.

Infectious disease expert Dr. Kristin Englund, who runs a center for long COVID patients at the Cleveland Clinic, said the Nature Medicine study mirrors what she sees at her clinic. Long COVID patients there include people who were vaccinated and received boosters.