Pfizer Inc. said on Thursday it would buy privately-held ReViral Ltd. in a deal worth as much as $525 million including milestone payments, to gain access to experimental drugs against the respiratory syncytial virus (RSV). The deal marks the U.S. drugmaker’s second acquisition in less than six months to boost its drug portfolio, after a $6.7 billion takeover of Arena Pharmaceuticals in December. RSV, a common respiratory virus that causes cold-like symptoms, is a cause of pneumonia in toddlers and the elderly. London-based ReViral has four RSV therapies in its pipeline, with lead product candidate sisunatovir and another treatment currently in mid-stage studies. Pfizer’s own RSV vaccine candidate is in late-stage studies now. “Currently, treatment options for RSV are extremely limited and focus primarily on supportive care,” said Annaliesa Anderson, chief scientific officer of Bacterial Vaccines and Hospital at Pfizer. Evan Seigerman, analyst at BMO Capital Markets, says the deal …