Pfizer Inc's shipment of Covid-19 vaccine to Mexico this week includes doses made in its US plant, the first of what are expected to be ongoing exports of its shots from the United States, a source familiar with the matter told Reuters on Thursday.The vaccine shipment, produced at Pfizer's Kalamazoo, Michigan plant, marks the first time the drugmaker has delivered abroad from US facilities after a Trump-era restriction on dose exports expired at the end of March, the source said.The US government has been under mounting pressure in recent weeks to provide surplus vaccines to other nations desperately in need as it makes swift progress vaccinating its own residents.