Three COVID vaccines have dramatically decreased in effectiveness over time, according to a new study.

According to a new study, as the Delta form of the coronavirus became the prevalent strain in the United States, all three COVID-19 vaccinations available to Americans lost some of their protective ability, with vaccine efficacy among a large group of veterans declining between 35 and 85 percent.

In early March, just as the Delta variety was acquiring a foothold across American communities, researchers analyzed the data of almost 800,000 U.S. veterans and discovered that the three vaccines were roughly similar in their effectiveness to prevent illnesses.

But that altered substantially during the next six months.

Moderna's two-dose COVID-19 vaccine, which was 89 percent effective in March, was just 58 percent effective by the end of September.

In the same time period, the effectiveness of shots made by Pfizer and BioNTech, which also used two doses, dropped from 87 percent to 45 percent.

Over the course of those six months, the protective power of Johnson & Johnson's single-dose vaccine dropped from 86 percent to just 13 percent.

The findings were published in the journal Science on Thursday.

The three vaccines fared better in terms of preventing COVID-19 deaths, but by July, when the Delta version began to cause a three-month rise in infections and deaths, the injections' efficacy on that front had also revealed significant discrepancies.

When compared to uninfected veterans of the same age, individuals who had a so-called breakthrough infection after receiving the Moderna vaccine were 76 percent less likely to die from COVID-19.

Veterans who received the Pfizer-BioNTech vaccination and then developed a breakthrough illness were 70% less likely to die than their unvaccinated counterparts.

When older vets who received a single J&J vaccination injection developed a breakthrough illness, they were 52 percent less likely to die than those who did not receive any shots.

A new study offers the most comprehensive comparison yet of how the three vaccines have performed across the nation this year. Boosters are recommended for those vaccinated, those with compromised immune systems and those with medical conditions that make them more vulnerable to a serious case of COVID-19. The study tracked 780,225 veterans of the U.S. armed forces from Feb. 1 to Oct. 1. Close to 500,000 of them had been vaccinated, while just under 300,000 had not. Hailing from across the country, all were cared for by the Department of Veterans Affairs' unified system, which provides healthcare to 2.7% of the U.S. population.

The CDC recommends that all recipients of the J&J vaccine get a booster. Dr. Barbara Cohn, the study's lead author, said in addition to its comparison of COVID-19 vaccines, the group's analysis provides "a lens for making informed decisions around primary vaccination, booster shots, and other multiple layers of protection."  Strong evidence of the vaccines' declining power should prompt even states and locales with highly vaccinated populations to consider retaining mask mandates, the authors said.  The study was conducted by a team from the Public Health Institute in Oakland, the Veterans Affairs Medical Center in San Francisco, and the University of Texas Health Science Center.