Soon after President Trump won the presidential election in November, British drugmaker GSK brought an unusual claim to federal prosecutors in Manhattan, according to people familiar with the matter. A senior GSK scientist, who formerly worked at rival Pfizer, had told GSK colleagues that Pfizer delayed announcing the success of its Covid vaccine in 2020 until after that year’s election. The scientist disputes that account of what he told colleagues. But prosecutors are taking a closer look at what GSK shared with them, which is potentially politically explosive. Trump for years has claimed that Pfizer sat on the positive results...