THIS IS THE FINAL REPORT OF THE 1999-2000 SEASON. WEEKLY REPORTS WILL RESUME IN OCTOBER 2000.

Synopsis: During week 20 (May 14 through May 20), none of the specimens tested for influenza virus by WHO and NREVSS laboratories were positive. State and territorial epidemiologists from 6 states reported sporadic influenza activity; none reported widespread or regional activity. The proportion of patient visits to sentinel physicians for influenza-like illness was within baseline levels of 0% to 3% in the United States overall and in all 9 surveillance regions. The proportion of deaths attributed to pneumonia and influenza was 6.6%. This percentage is below the epidemic threshold for week 20.

During the current season, the overall national percentage of respiratory specimens positive for influenza peaked at 33% during week 51. During the previous 3 seasons (1996-97, 1997-98, and 1998-99), the peak percentages of respiratory specimens testing positive for influenza viruses ranged from 28% to 34%. For this season, the percentage of overall patient visits for influenza-like illness peaked at 6% during week 52. During the previous 3 years, the peak percentages for such visits ranged between 5% and 7%. The proportion of deaths attributed to pneumonia and influenza (P&I) peaked at 11.2% during week 3. During the previous 3 years, P&I mortality levels peaked between 8.8% and 9.1%. This season's P&I figures must be interpreted with caution because important changes have taken place in this year's case definition that may be contributing to higher estimates of P&I mortality than in previous years