A new study published Friday by the Centers for Disease Control and Prevention (CDC) has found that about 75% of Massachusetts residents infected in the COVID pandemic were fully vaccinated, with four of them getting hospitalized, according to CNBC.

The new study, published in the CDC’s Morbidity and Mortality Weekly Report, also found that infected people who were fully vaccinated carry as much of the virus in their noses as unvaccinated people, potentially spreading it to other individuals.

CDC Director Dr. Rochelle Walensky said in a statement, “This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation. The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit the virus to others, including their unvaccinated or immunocompromised loved ones.”

On Tuesday, the CDC once again updated its guidelines, recommending fully vaccinated people who reside in areas with high infection rates resume wearing facemasks indoors.

The Delta variant, which was first identified in India, continues to affect unvaccinated people in the United States.

Dr. Walensky said some vaccinated people could be carrying higher levels of the virus than previously understood and are potentially transmitting it to others, adding that the Delta variant behaves “uniquely differently from past strains of the virus.”

The Delta strain has already been the dominant form of the variant in the United States, which is highly contagious than the common cold, the 1918 Spanish flu, smallpox, Ebola, among others, according to the CDC.

While studies have shown that the currently available vaccines do not work well against the Delta variant as they did against other strains, public health officials say they are still highly effective, in preventing severe illness and death.

The officials reported this week that nearly 97% of new hospitalizations and 99.5% of COVID deaths in the U.S. are among unvaccinated.

However, the CDC said the study has limitations, noting that as population-level vaccination coverage increases, vaccinated persons are likely to represent a larger proportion of COVID cases, per CNBC.

In addition, the agency said, “asymptomatic breakthrough infections might be underrepresented because of detection bias.”

Furthermore, the CDC said the study is “insufficient” to conclude about the efficacy of the FDA-approved vaccines against COVID, including the highly transmissible Delta variant. The article originally appeared on CNBC.