Re-emerging of H5Nl severe outbreaks in vaccinaied chickens housed in poultry farms in Sharkia governorate in Egypt was observed starting from October 2007 and continued for 3 months until control measures have been taken in such farms. besides multiple vaccination during this period were applied using Reassortant H5N1 and 6 types Of H5N2 vaccines. In the present study, 9922 serum samples were collected from vaccinated chickens including some broiler breeders and commercial layers from the period Of December 2006 to February 2008 and tested for the immune response to the multiple vaccination with different H5Nl and H5N2 avian influenza vaccines (Reassortant H5N1 and 6 types of H5N2 vaccines) by hemaggtutination inhibition (HI) test using heterologous H5 antigen which represent non of the vaccine antigens. The samples were collected from 578 houses mostly commercial layers. H5Nl viral nucleic acid was also detected in swab samples collected from mortalities occurred in some of vaccinated birds after vaccination policy has been applied in Egypt at March 2006. The viral RNAs of the detected H5Nl circulating viruses between Feb 2006 and Feb 2008 in Sharkia governorate were sequenced. Results of nucleotide sequence analysis of I I detected viruses confirm the existence and circulation of the H5Nl in Sharkia governorate along the period of study and no H7 viruses were detected. The serum samples collected from different farms were divided into 4 groups based on the date of collection. HI results showed that in the first group (samples collected between December 2006 to March2007) 43% of the tested samples were of HI titers of 6 log 2 oc more which we propose such titer as a protective titer against H5Nl virus. In group 2 (samples collected between April and July, 2007), titers in 36.4% of the tested samples were protective. In group 3 (samples collected between August to November, 2007).