Kaiser Permanente employees in California, including about 4,700 in the North Bay, voted to stay with their current trade union, rather than switching, according to results announced Thursday afternoon by the National Labor Relations Board.

The agency said it was the largest mail ballot election it has ever refereed.

Out of 32,588 mail ballots deemed valid from more than 45,000 employees eligible to vote, 18,844 were cast for Service Employees International Union-United Healthcare Workers West (SEIU-UHW) and 13,101 for National Union of Healthcare Workers and California Nurses Association (NUHW-CNA). Of the remaining ballots, 334 were for no union, 520 voided and 309 challenged. Each side has seven days to officially dispute the election.

The votes received by the labor regulator over three weeks then verified and counted yesterday and today.

The election was a rerun of a previous mail ballot election held Sept. 13 through Oct. 7, 2010. A labor board judge set aside that election and called for a new one, based on election-related conduct by SEIU-UHW.