Shares of Novavax Inc. /zigman2/quotes/202614340/composite NVAX +0.11% were down 0.9% in trading on Tuesday, the day after the company said a study evaluating co-vaccination against COVID-19 and the flu demonstrated similar efficacy to administering Novavax's experimental COVID-19 shot alone. "Separate health care visits to cover both COVID-19 and influenza vaccinations will be burdensome," a Novavax executive said in a news release. As part of the Phase 3 clinical trial for its coronavirus vaccine in the U.K., the company also enrolled 431 participants in a sub-study that gave participants both the COVID-19 vaccine and the flu shot. In the preprint , published Sunday, the researchers found that the immunogenicity of the flu shot was the same, while there was a "modest decrease" in the immunogenicity of the COVID-19 shot. Novavax also announced this week that its coronavirus vaccine had an efficacy rate of 90.4% when it comes to preventing symptomatic infection in roughly 30,000 participants enrolled in the Phase 3 clinical trial in the U.S. Novavax's stock has soared 86.2% so far this year, while the S&P 500 /zigman2/quotes/210599714/realtime SPX -0.91% is up 13.3%.