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Flu Update for Week 49 ending December 9th 2023

Tracking Seasonal influenza activity

Flu Update for Week 49 ending December 9th 2023.  Currently the flu is increasing in most parts of the country.  The southeast, south-central, and west coast areas of the country reporting the highest levels of activity.

Flu Virus Summary

We are seeing Influenza A at 82%.  The rest is Influenza B and a few H2 variants at 18%.

The quadrivalent Vaccine appears to protect from the current Flu viruses

Clinical Laboratories Reports Summarized

The results of tests performed by clinical laboratories nationwide are summarized below. Specifically, data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

results of tests from Clinical Laboratories
Week 49
Data Cumulative since
October 1, 2023
(Week 40)
No. of specimens tested 90,439 810,104
No. of positive specimens (%) 9,212 (10.2%) 36,366 (4.5%)
Positive specimens by type
Influenza A 7,570 (82.2%) 27,751 (76.3%)
Influenza B 1,642 (17.8%) 8,615 (23.7%)



INFLUENZA Virus Isolated

Clinical Laboratories Flu results December 9


Public Health Laboratories Reports Summarized

Tests results performed by public health laboratories nationwide are summarized below.  Currently, the data from public health laboratories used to monitor the proportion of circulating influenza viruses.   Therefore, this data summarizes the influenza subtype/lineage variants.

results of tests from Public Health Laboratories
Week 49
Data Cumulative since
October 1, 2023
(Week 40)
No. of specimens tested 3,060 32,227
No. of positive specimens 971 6,596
Positive specimens by type/subtype    
         Influenza A 806 (83.0%) 5,345 (81.0%)
Subtyping Performed 500 (62.0%) 4,209 (78.7%)
            (H1N1)pdm09 374 (74.8%) 3,519 (83.6%)
             H3N2 126 (25.2%) 690 (16.4%)
             H3N2v 0 (0.0%) 0 (0.0%)
Subtyping not performed 306 (38.0%) 1,136 (21.3%)
        Influenza B 165 (17.0%) 1,251 (19.0%)
Lineage testing performed 95 (57.6%) 1,000 (79.9%)
            Yamagata lineage 0 (0.0%) 0 (0.0%)
            Victoria lineage 95 (100%) 1,000 (100%)
Lineage not performed 70 (42.4%) 251 (20.1%)
INFLUENZA Virus Isolated

Public Laboratories reporting to CDC Flu results December 9


Nationwide during Week 49, 4.4% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has increased compared to Week 48 and is above the national baseline of 2.9% for the sixth consecutive week.  Link back to our last Flu Post.


LINet is a system that monitors visits for respiratory illness  called ILI

The Current State map on LiNet Weekly Influenza Surveillance Report