June 16, 2021 -- Giving a COVID-19 vaccine at the same time as a seasonal flu vaccine appears safe and effective, according to a new study.

Overall, the Novavax vaccine (Novavax) has shown 89.8% efficacy in an ongoing, placebo-controlled phase III study, the first to test how people react to getting both shots simultaneously When the researchers gave a smaller group of 431 volunteers from the same study an influenza shot at the same time, efficacy dropped slightly to 87.5%.

"These results demonstrate the promising opportunity for concomitant vaccination, which may lead to higher vaccination rates and further protection against both viruses," says study co-author Raja Rajaram, MD, a medical affairs lead at Seqirus, the company that supplied the flu vaccines for the research.

The research was published online June 13, 2021 as a MedRxiv preprint, and has not been peer reviewed

"With these COVID-19 vaccines, there are essentially no concurrent use studies," Paul A. Offit, MD, says. tells Medscape Medical News.

Traditionally, how a new vaccine might interact with existing vaccines is studied before the product is cleared for use. That was not the case, however, with the COVID-19 vaccines made available through emergency use authorization.

The researchers found no major safety concerns with combining the vaccines, Rajaram said. In addition to safety, the current study aimed to see if either vaccine changed the effectiveness of the other.

"It's a small study, but it's certainly encouraging to know that there didn't seem to be a big decrease in [effectiveness] either way and the safety profile was similar. Not identical, but similar," addsOffit, the director of the Vaccine Education Center at Children's Hospital of Philadelphia.

Some adverse events were more common in the co-administration group. For example, soreness at the injection site was reported by 70% compared to 58% for those who got the COVID-19 shot alone. The same was true for pain at the injection site, 40% vs 29%; fatigue, 28% vs 19%; and muscle pain, 28% vs 21%.

Rates of side events that required medical care, and serious adverse events were low and well-balanced between groups.

A Boost for Booster Vaccines?

The research could carry implications for future COVID-19 booster shots, Gregory A. Poland, MD, an internist and part of the Vaccine Research Group at Mayo Clinic in Rochester, MN, says.

"Overall, the study results are reassuring and of potential practical importance if we have to give booster doses. It will make it easier to give them both in one visit," says Poland, who was not affiliated with the research.

Although Novavax could be positioning itself as a logical choice for a COVID-19 booster based on the findings, Offit believes it is more important to focus on having more COVID-19 vaccine options available.

"There may be, as we say at the track, 'courses for horses,'" he says, meaning that different vaccines may be better suited for different situations.

"It's likely we're going to find these vaccines have different safety profiles, they may have different populations for whom they work best, and they may have differences in terms of their long-term durability," he adds. Also, some may prove more effective against certain variants of concern.

The Novavax vaccine would add a new class of COVID-19 vaccine to the mRNA and adenovirus vaccines. It is a recombinant spike protein vaccine.

"I think the more vaccines that are available here, the better," Offit says.