Virus expert Joseph Hyser has been fully vaccinated since the end of January, and he's not really worried that his COVID-19 protection from the Pfizer vaccine has waned much since then.

Yet, when he got an email last month asking if he'd be interested in getting a third dose booster shot - this time from Moderna - he did not hesitate to say yes.

The virologist at Baylor College of Medicine signed right up, and took the elevator down six floors to Baylor's Vaccine Research Center, where he offered up his arm for a third jab.

"Oooh I got boosted!" he remembered bragging to family and friends afterwards, who were nonplussed about his unique booster shot experience.

Perhaps they didn't realize that Hyser is one of the first Americans to participate in a groundbreaking COVID-19 mix-and-match study, aimed at assessing whether boosters work, and whether different COVID-19 shots are safe - and possibly more effective - when they're mixed together.

Though Hyser says he experienced "similar types of side effects" from both Moderna's and Pfizer's mRNA shots, he said the side effects were "a little bit more obvious with the Moderna."