• The highly contagious delta variant will lead to an increase in breakthrough infections among the fully vaccinated as people begin moving indoors, Moderna said.
• While Moderna's two-dose vaccine remains "durable" six months after the second shot, immunity against the virus will continue to wane and eventually impact vaccine efficacy, it said.
• "Given this intersection, we believe dose 3 booster will likely be necessary prior to the winter season," Moderna wrote.

The highly contagious delta variant will lead to an increase in breakthrough infections among the fully vaccinated as people begin moving indoors after the summer, Moderna said Thursday.

While Moderna's two-dose vaccine remains "durable" six months after the second shot, immunity against the coronavirus will continue to wane and eventually diminish vaccine efficacy, the company said in slides that accompanied its second-quarter earnings report.

The company said its vaccine is 93% effective six months after the second dose. By comparison, Pfizer and BioNTech said their vaccine efficacy declined to around 84% after six months.

"Given this intersection, we believe dose 3 booster will likely be necessary prior to the winter season," Moderna wrote.

The warning by Moderna comes as the delta variant continues to spread in more than 100 countries, including the United States. Delta, the dominant form of the disease in the U.S., is more transmissible than the common cold, the 1918 Spanish flu, smallpox, Ebola, MERS and SARS, according to the Centers for Disease Control and Prevention.

For some Americans, concerns about vaccine efficacy have intensified with the rise of the variant, which may cause more severe disease than the original coronavirus. Some people have gone as far as seeking out a supplemental dose, which is not yet recommended by the CDC. This week, San Francisco health officials said they are allowing patients who received Johnson & Johnson's vaccine to get a second shot produced by either Pfizer-BioNTech or Moderna.

Drugmakers have said for months they expect people will eventually need booster shots and perhaps extra doses annually, just like for the seasonal flu.

Moderna said Thursday results from a phase two trial found a booster dose of its vaccine produced a "robust" antibody response against three variants, including delta.

The CDC and the World Health Organization say booster doses are not needed at this time, citing lack of data. In fact, the WHO on Wednesday called on wealthy nations to stop the distribution of Covid booster shots to give the world a chance to meet the WHO's goal of vaccinating 10% of the population of every country by October.

"We need an urgent reversal from the majority of vaccines going to high-income countries, to the majority going to low income countries," WHO Director General Tedros Adhanom Ghebreyesus said.

The move comes after Israel announced the country would give booster doses to its elderly population. The Dominican Republic has also been administering booster doses to its population, while neighboring country Haiti only recently secured its first batch of vaccine doses.

People in the U.S. are also finding ways to secure booster shots.

–CNBC's Rich Mendez contributed to this report.