The researchers looked at patient visits to hospitals and urgent care centres.
They estimated how well Pfizer or Moderna booster shots prevented Covid-related visits to emergency departments and how well the vaccines prevented hospitalisations.
Researchers also found that during the time that the omicron variant has been predominant, vaccine effectiveness against outpatient visits was 87 percent in people who had gotten a booster two months earlier.
However, this fell to 66 percent four months after.
Vaccine effectiveness against hospitalisation fell from 91 percent in two months to 78 percent by the fourth month.
Health experts expect protection from the vaccines to wane.