Influenza Surveillance

Preparing for Seasonal and Pandemic Influenza

CDC and public health professionals all over the world protect the public from seasonal and pandemic influenza viruses.

CDC and its public health partners conduct global influenza surveillance all year by tracking circulating influenza viruses, performing laboratory tests to determine how circulating influenza viruses are changing, and monitoring for the emergence of new influenza viruses in the human population. Year-round global influenza surveillance allows CDC and other public health professionals to recommended which influenza viruses to include in the formulation of seasonal flu vaccines for the Northern and Southern Hemispheres, and to remain vigilant for the emergence of novel influenza viruses that could trigger a pandemic.

Phylogenetic Tree for Influenza B Viruses, 1969

Phylogenetic Tree for Influenza B Viruses, 1969

Comparing Influenza

CDC continuously compares the genes of circulating influenza viruses to the genes of older influenza viruses. Through this process of comparing genetic sequences, called genetic characterization, CDC can map out how flu viruses are “related” to one another.

The relative differences between groups of influenza viruses can be visually seen on a “phylogenetic tree.” Phylogenetic trees for influenza viruses are like family trees for people. These trees show how closely “related” viruses are to one another.

Each sequence from a specific influenza virus has its own branch on the tree. The degree of genetic difference between viruses is represented by the length of the horizontal lines (branches) in the phylogenetic tree. The further apart viruses are on the branches of a phylogenetic tree, the more genetically different the viruses are.

Dr. Hunein "John" Maassab in his lab

Dr. Hunein “John” Maassab in the lab

Tracking Influenza in the Past:  Influenza Virus Identification Cards from Dr. John Maassab’s Lab

Dr. Hunein "John" Maassab, a Syrian immigrant who came to the U.S. in the late 1940s, dedicated his career to studying influenza. Beginning in the 1960s, Dr. Maassab worked to develop a live attenuated influenza vaccine at the University of Michigan Department of Epidemiology. In 2003, the Food and Drug Administration, after several years of studies and vaccine trials, licensed the nasal-spray flu vaccine that Dr. Maassab spent years developing. 

Dr. Maassab kept and cataloged information about older influenza viruses throughout his career. Each card contains information about a particular group of viruses.

The Influenza Viruses
Influenza Surveillance