With the Delta variant in play, federal health officials are analyzing the latest numbers to look at the frequency and seriousness of COVID-19 breakthrough infections. Will we need booster shots? And if so, when? New studies are underway to find answers.
Moderna released new trial results showing after six months their vaccine remains 93% effective against symptomatic illness.
"We're seeing durable protection against COVID-19 which is actually really welcome given everything we're facing," said Dr. Stephen Hoge, president of Moderna.
Moderna stopped collecting data before the Delta variant emerged in the U.S., but Hoge said the real world evidence demonstrates it does offer protection against the variant.
"Knowing whether or not that will hold up for instance through the winter is not something we have information on right now," he said.
The Centers for Disease Control and Prevention's most recent numbers show 99% of COVID-19 deaths and 95 percent of hospitalizations are among the unvaccinated, but CDC Director Dr. Rochelle Walensky said that data was gathered between January and June 2021.
"We are actively working to update those in the context of the Delta variant," Walensky said.
So what is the timeline for booster shots? A senior government official tells ABC News the FDA is expected to have a plan for booster vaccines for immunocompromised Americans in the next few weeks.
Meanwhile, Pfizer said it will submit its booster data to the FDA by August 16.
Also, new research suggests having a flu shot might help boost immunity and help limit severe COVID symptoms.
"It could be the nonspecific juicing up of the immune system. That's one of the things that you always think about. But, I'd be careful about that. The data on that are not strong," said Dr. Anthony Fauci, chief medical advisor to President Joe Biden.
Researchers at twelve sites across the country are studying the safety and effectiveness of different booster shots and vaccines. Can you mix Moderna with Pfizer shots, or would it be better to receive a Johnson & Johnson booster, or vice-versa?
"So, we are studying all of the different combinations in order to answer that question," said pediatric infectious disease specialist Dr. Judy Martin of the University of Pittsburgh Medical Center.
Volunteers will provide blood samples so researchers can study immune responses. Scientists say it's important information to have as move COVID-19 variants like Delta are identified.
"The concern is that at some point our current vaccinations might not protect us as well as they are doing right now for the variants," said Martin.
Researchers will follow participants for a year, but hope to present initial results in the coming weeks. Vaccine makers say they're planning for the worst.
"What we're gonna do is prepare options, but we are preparing options because we think they're needed," said Hoge.