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.