The pre- and post immunization HI antibodies were tested at the Central Virology Laboratory of the Israeli Ministry of Health using the HI test according to a standard WHO procedure 16. Sera will be separated, code labeled, and stored at -20°C until tested. Sera will be treated with receptor destroying enzyme cholera filtrate to remove non-specific inhibitors, and with Turkey red blood cells to remove non-specific agglutinins. The treated sera will be tested by HI test against the three antigens included in the vaccine: A/California (CAL), A/Wisconsin and B/Malaysia. The working dilution (test dose) of each antigen contained four haemagglutinin units in 25 µl of antigen. Test doses will be diluted in phosphate buffered saline (PBS) and added to serial dilution of antiserum. The haemagglutinin inhibition titer will be determined as the highest dilution of serum that completely inhibits haemagglutination of red blood cells.