The effectiveness of COVID-19 vaccines dropped dramatically after six months, according to a large study. The new study of U.S. military veterans found that vaccine efficacy against COVID-19 infection plummeted as much as 73% in six months.

The study – published in the journal Science – found that protection from COVID-19 infection provided by the three vaccines approved in the U.S. fell from 87.9% in February to 48.1% in October.

The study examined data from the Veterans Health Administration of 498,148 fully vaccinated veterans. Moderna's two-dose COVID-19 vaccine went from being 89% effective in March to 58% effective in September. The Pfizer/BioNTech vaccine had an 87% efficacy against coronavirus in March and dropped to only 45% in September. In the same six-month period, the Johnson & Johnson vaccine plummeted from 86% effectiveness to a mere 13%.

The study also examined data from nearly 300,000 unvaccinated veterans to determine the efficacy of the vaccine against death. The study found that those aged 65 and older who got the Moderna vaccine were 76% less likely to die of COVID-19, 70.1% risk of death for those inoculated with the Pfizer vaccine, and 52.2% chance of death with those who took the one-dose J&J shot.

The authors of the study said, "Benefits of vaccination in reducing risk of SARS-CoV-2 infection and death are clearly supported by this study of more than 780,225 U.S. Veterans. Those fully vaccinated had a much lower risk of death after infection."

The Los Angeles Times noted, "Because these were veterans, the study population comprised six times as many men as women. And they skewed older: about 48% were 65 or older, 29% were between 50 and 64, and 24% were younger than 50."

The study said that vaccines are effective in preventing infection and death, but should be coupled with wearing face masks and social distancing.

"Our findings support the conclusion that COVID-19 vaccines remain the most important tool to prevent infection and death," the study said. "Vaccines should be accompanied by additional measures for both vaccinated and unvaccinated persons, including masking, hand washing, and physical distancing."

Because of the declining vaccine efficacy over time, the study's authors also called for public health interventions, including "strategic testing for control of outbreaks, vaccine passports, employment-based vaccine mandates, vaccination campaigns for eligible children as well as adults, and consistent messaging from public health leadership in the face of increased risk of infection due to the Delta and other emerging variants."

Dr. Barbara Cohn – the lead author of the study – stated, "Our study gives researchers, policymakers and others a strong basis for comparing the long-term effectiveness of COVID vaccines, and a lens for making informed decisions around primary vaccination, booster shots, and other multiple layers of protection, including masking mandates, social distancing, testing and other public health interventions to reduce chance of spread."

Cohn added, "Breakthrough infections are not benign, but also by the strong evidence that vaccination still protects against death even for persons with breakthrough infections, compared to persons who become infected and are not vaccinated."

The Centers for Disease Control and Prevention (CDC) recommends those who received the Moderna or Pfizer shot and are eligible for a booster wait until at least six months after the second dose to get a booster shot. The CDC advises those who received the Johnson & Johnson COVID-19 vaccine to get a booster shot two months after the original vaccination.

The study was conducted by researchers from the Public Health Institute in California, the Veterans Affairs Medical Center in San Francisco, and the University of Texas Health Science Center.