Moderna said Thursday that its COVID-19 vaccine maintained 93 percent efficacy six months after the second dose but said that a booster shot will likely still be needed before the winter.
The 93 percent efficacy after six months announced by Moderna ahead of an earnings call on Thursday is a positive sign and compares to 84 percent efficacy after that time for the Pfizer vaccine.
Still, Moderna CEO Stephane Bancel said on CNBC on Thursday that the data was collected before the delta variant became prevalent in the United States, meaning the equation could change now that the delta variant is widespread.
"We are pleased that our COVID-19 vaccine is showing durable efficacy of 93% through six months but recognize that the Delta variant is a significant new threat, so we must remain vigilant," Bancel said in a statement.
While there is strong data for six months, the company said in a presentation that it believes antibody levels will "continue to wane and eventually impact vaccine efficacy."
It added that the combination of delta, fatigue with wearing masks and people moving indoors as the weather gets colder will cause an "increase of breakthrough infections in vaccinated individuals."
"Given this intersection, we believe dose 3 booster will likely be necessary prior to the winter season," the company said.
Phase 2 trials have shown "robust" immune responses from a booster shot, including against delta, the company said.
Pfizer has also said it thinks a booster shot will be needed.
The idea of giving third doses in wealthy countries like the U.S. while many people around the world wait for even a first dose has drawn pushback from the World Health Organization.
U.S. health officials have not yet announced that booster shots are needed, but they are leaving open that possibility, particularly for vulnerable groups like the immunocompromised or the elderly.