Moderna (MRNA  ) warned on Thursday that the highly contagious Delta variant will likely lead to an increase of breakthrough infections among fully vaccinated people later this year, echoing concerns recently raised by the U.S. Centers for Disease Control and Prevention (CDC).

The biotech noted that while its COVID vaccine's efficacy remains high six months after the second dose, immunity against the coronavirus will eventually lessen over time, Moderna said in slides that accompanied its second quarter earnings report.

Moderna said that its vaccine is about 93% effective six months after the second dose, falling about two percentage points from its original efficacy rating. Still, the shot is very effective, beating the six month efficacy of competitor Pfizer (PFE  )-BioNTech (BTNX  ), with the pair's shot dropping from 95% to 84% effectiveness after six months, according to Pfizer's second quarter earnings report.

"We believe that increased force of infection resulting from Delta...and seasonal effects [people moving indoors during the winter months] will lead to an increase of breakthrough infections in vaccinated individuals," Moderna said in the slide show. "Given this intersection, we believe dose 3 booster will likely be necessary prior to the winter season."

Moderna's warnings comes soon after the CDC reversed course on its masking guidance for fully vaccinated Americans as the Delta variant continues to rapidly spread throughout the United States. The federal health agency now recommends that fully vaccinated people wear masks indoors in areas of high or substantial COVID infection rates. The CDC also found that the Delta variant is more transmissible than other highly infectious diseases like the common cold, Ebola, MERs, SARS and Chickenpox, and may cause breakthrough infections among the fully vaccinated that can be spread to others.

Drugmakers and health experts have been forecasting for months that fully vaccinated people will eventually need booster shots and potentially another dose of the vaccine annually, similar to the vaccine for the seasonal flu. The CDC and the World Health Organization (WHO), however, do not recommend either booster shots or a third dose of vaccine at this time.

Nevertheless, Moderna is currently studying a third booster shot of its vaccine in clinical trials of adult volunteers who have received both full doses of the biotech's vaccine. The company said on Thursday in its second quarter earnings report that results from a Phase 2 trial found a booster dose of its vaccine produced a "robust" antibody response against multiple known COVID variants, including Delta.

In the clinical trial, Moderna tested a 50-microgram dose of three different vaccine booster candidates, with the shots producing positive immune responses against multiple variants, with antibody levels nearing those seen in prior studies of unvaccinated people who received two 100-microgram doses of the company's original COVID vaccine.

Still, the WHO is advising wealthy nations to hold off on giving booster shots to their populations through September. Instead, the global health agency wants the world to focus on vaccinated at least 10% of every nation's population by October, which the WHO sees as the key to eventually ending the global threat of the coronavirus.