The highly contagious Delta variant will lead to an increase in breakthrough infections in those who are fully vaccinated as people begin to exercise indoors after the summer, Moderna said Thursday.

While Moderna’s two-dose vaccine remains “stable” for six months after the second vaccination, immunity to the coronavirus will continue to decline and ultimately affect the vaccine’s effectiveness, the company said in the slides that its earnings report for the second quarter were attached.

The company said its vaccine was 93% effective six months after the second dose. By comparison, Pfizer and BioNTech reported that their vaccine effectiveness decreased to about 84% after six months.

“Given this overlap, we believe a dose 3 refresh will likely be needed before the winter season,” wrote Moderna.

Moderna’s warning comes as the Delta variant becomes more widespread in more than 100 countries, including the United States. Delta, the predominant form of the disease in the United States, is more transmissible than the common cold, 1918 Spanish flu, smallpox, Ebola, MERS, and SARS, according to the Centers for Disease Control and Prevention.

A healthcare worker treats a patient in a negative pressure room in the Covid-19 Intensive Care Unit (ICU) at Freeman Hospital West in Joplin, Missouri, Tuesday, August 3, 2021.

Angus Mordant | Bloomberg | Getty Images

For some Americans, concerns about the effectiveness of the vaccine have increased with the advent of the variant, which can spread more easily and cause more severe illness than the original coronavirus. Some people have even gone so far as to look for an extra dose not yet recommended by the CDC. This week San Francisco health officials announced that they would allow patients who received the Johnson & Johnson vaccine to have a second vaccination from Pfizer-BioNTech or Moderna.

Drug makers have been saying for months that they expect people to need booster shots and perhaps additional doses annually at some point, just like they did with seasonal flu.

Moderna said Thursday that results from a Phase 2 study showed that a booster dose of its vaccine elicited a “robust” antibody response against three variants, including Delta.

The CDC and World Health Organization say booster doses are not currently required due to a lack of data. In fact, on Wednesday the WHO called on wealthy nations to stop distributing Covid booster vaccinations to give the world a chance to meet the director general’s goal of vaccinating 10% of each country’s population by the end of September.

“We need an urgent reversal of the majority of vaccines going to high-income countries and the majority to low-income countries,” WHO Director General Tedros Adhanom Ghebreyesus said at a press conference.

The move comes after Israel announced that the country will be giving booster doses to its elderly population. The Dominican Republic has also given its population booster doses, while neighboring Haiti recently secured its first vaccine doses.

People in the US are also finding ways to get booster vaccinations.

– CNBC’s Rich Mendez contributed to this report.