A failed vaccine would have a catastrophic impact to the financial position of Biopharm and his personal wealth, which might give him the clearest motive for murder and keeping such news quiet.
Conveniently using deadly nightshade for his Parkinson’s?!
His family would absolutely support his alibi during time of the murder.

Backstory…

Damon’s father, Arthur Whittaker, is a well-known real estate mogul and as a result had rooted the family in Knightsbridge, one of the most expensive neighborhoods in the world.

His mother, Olivia (Walters), of the Walter Reynolds Energy Company Walters, met Arthur at a Sotheby’s auction when some the family’s private collection was being offered.

Damon also has a younger sister, Paige (33), who more than willingly adopted the whimsical pursuits of her mother and other society people…gossiping, shopping and partying going.
Being a “prized catch”, she has never married, but enjoys being pursued.

Everyone always recognized his parents and is why he wanted out of the family’s shadow, and to establish his own identity and worth in a completely different industry.

While pursuing this path at Cambridge, he met Rebecca Rogers at a party. Soon after, Rebecca found out she was pregnant, and told Damon. Despite his upbringing, his first inclination was to help Rebecca and be responsible. But being a Whittaker also had its drawbacks, one of which was not anonymity, and word quickly got to his family.

After a thorough background check of Rebecca and finding her to be “common” and not to the Whittaker standards, Damon’s family swiftly imposed restricted access and interactions with Rebecca, against his wishes. At the same time, an anonymous substantial deposit of money was placed in her account.

After this episode, Damon never could truly shake this event, but continued on his way to realizing his goal of independent success, until the recent discovery that his vaccine didn’t work.

Additionally, he was recently diagnosed with early-stage Parkinson’s disease, which he has keep from his family and media for fear of the financial implications.

The fact that he is Meghan’s biological father and recently amended his trust to leave all of his financial assets to her makes it extremely difficult to believe he would then kill her.