Shares of Gilead Sciences (GILD) - Get Report, a top contender in the race to develop a coronavirus vaccine, fell sharply Monday after a downgrade by analysts at RBC Capital Markets.

Gilead's stock price dropped 5.9% to $75.49 a share after a relatively modest downgrade by RBC to outperform, a decline also likely fueled as well by the the general market rout following a price war in oil. Gilead had previously been rated as a top pick by RBC.

Along with dropping their rating on Gilead a notch, analysts at RBC kept their price target on the biotech steady at $86 a share.

While RBC noted it expects "further upside" from Gilead, analysts at the firm wrote they are growing wary of the relatively high valuation of the drug development company's share price, currently trading at 11.8 times its estimated 2020 earnings.

Gilead's stock is up from $65.23 a share at the start of the year, compared to a double-digit decline in the Standard and Poor's 500 index over the past month.

RBC said it has been high on "underappreciated sustainability" of Gilead's 11 different HIV medications, the biotech's approach to business development, and its "undervalued pipeline/antiviral prowess."

That said, "our bullish thesis  ... is beginning to play out," analysts at the firm wrote.

Gilead has launched Phase III testing on Remdesivir, developed as a possible Ebola treatment, on coronavirus patients in China, looking at two different groups, one with severely ill patients, the other with those only moderately ill.

RBC sees the coronavirus drug trials as a way Gilead can maintain its position as a "defensive play" amid the current market tumult.