NEW YORK (NewsNation Now) — Plastic shields were once thought to be effective in stopping the spread of COVID-19 but a new study says they may actually make things worse.

“Initially, we thought the plastic barriers would be helpful, but what we’re now learning is that they might actually block air from ventilating and flowing freely,” said Dr. Purvi Parikh, an immunologist at NYU Langone Health.

Researchers say that is actually part of the formula for catching a respiratory illness.

The new theory suggests that in some cases the shields can cause dead air — spaces where it isn’t circulating. That could make the viral load rise and leave the area concentrated with COVID-19.

A study published in June and led by researchers from Johns Hopkins University showed desk screens in classrooms were associated with an increased risk of coronavirus infection. There’s also fear that physical barriers can lull people into a false sense of security, causing them to forget the basics.

“That is masking, distancing, good ventilation in those air-spaces when you’re indoors. Those are what have been shown to really help reduce the risk of transmission of SARS CoV-2,” said Dr. Dana Hawkinson, an infectious disease physician at the University of Kansas Health System.

The good news, doctors say, there is still a barrier that works — masks. Doctors say next to a vaccine wearing one is the best defense.