Serum samples available from 4 critically ill patients exhibited HI titres of 40-320. At admission, the geometric mean titres (GMT) of anti-rpH1N1-09-HA-IgG antibodies (ELISA) were significantly lower in the mild infections (800, 95% CI values 571-1120) than the critically ill patients (4996, 95% CI values 3970-6288) (p,0.0001). The last blood sample collected 4-11 days later from 11 critically ill patients showed .4 fold rise in antibody titres. IgG1 was the exclusive isotype in both the categories. 