Europa Conference League
Fans threw plastic cups and other objects at Biraghi as he prepared to take a corner during the match in Prague and he had to receive medical attention after being struck on the head.
The Hammers are through to a European club competition final for the first time in almost half a century, in which they will face Italian side Fiorentina in Prague on June 7.
Reaching the European final is the bright spot of a poor season for West Ham, with Moyes' future looking uncertain at one point as they plunged into the relegation zone earlier this year, though they have since climbed to the brink of safety.
The Italians made their first European final in 33 years and will face West Ham United in the showpiece match in Prague on June 7 after the Londoners beat AZ Alkmaar in their semi-final.
The Swiss Super League side are now in a solid position to become the first team from their country to reach a European final.
The Scot has taken the London club to a European semi-final for the second successive season and hopes to go one better this time by getting past Dutch club AZ Alkmaar.
Coach Antonio Conte confirmed earlier that there had been 13 positive cases at Tottenham, eight players and five members of staff.
Union Berlin's next home game in the Europa Conference League is on Nov. 4 against Feyenoord.
Many clubs refused to release players to 'red list' countries but Spurs' Cristian Romero, Giovani Lo Celso and Davinson Sanchez went on international duty.