Epidemiology of Influenza

The Flu Basics

• The flu is contagious and can range from mild to deadly

• Each year between 5% and 20% of the US population contracts the flu– During pandemics the rate is between 25% and 50%

• Annually there are more than 200,000 hospitalizations due to the flu

• Approximately 36,000 people die a year from the flu in the US– Most of which are the elderly, the young, and people

with certain health conditions

The Flu is Unpredictable

• Outbreaks have been known to occur in three patterns– pandemics every 30 to 40 years

• high mortality

– epidemics much more frequently• lower mortality

– mild sporadic outbreaks

Morbidity and Mortality

• Recognized since at least the time of Elizabeth I of England

• Hard to quantify morbidity and mortality since exact diagnosis relies on laboratory confirmation

Morbidity

• Quantification of morbidity has required different approaches– community and family studies have made major

contributions

• Morbidity is most pronounced in children and young adults– the diseases are self-limited, they are often severe

• Each year must be considered an influenza year in lieu of the fact that the size of the outbreak varies since it can be documented annulally

Mortality

• Excess mortality has been documented since 1889

• Various methods have been developed to provide estimates of the mortality associated with influenza– usually predicated on establishing expected baseline

rates of mortality– deaths in excess of these rates are calculated and

attributed to the circulating influenza virus• Groups at high risk of mortality have been

defined as the elderly, those with chronic conditions, and those in institutions (prison)

http://nmhm.washingtondc.museum/collections/archives/agalleries/1918flu/Reeve3203.jpg

Vectors (bird, mammalian, human)

http://www.hopkins-cepar.org/bin/c/e/bird_flu_transmission_chart.gif

Transmission

• Influenza is highly contagious – it is easily transmitted through contact with droplets

from the nose and throat of an infected person released while coughing or sneezing

• The disease infects the respiratory tract• It often breaks out as an epidemic

– It can quickly spreads from town to town and country to country with the help of modern transportation

• Epidemics generally exist in an area for a period of four to six weeks before it eases off

• Influenza can make many people ill in a short period of time– onset of symptoms range from 18 to 72 hours.

Emerging Flu (H5N1)

• subtype of Influenza A virus– “H” in H5 refers to the type of hemagglutinin

antigen on the viral envelope – N1 refers to the neuroaminidase enzyme that

helps get through the mucal layer and in the budding of new virus particles

• H5N1 emerged in humans in 1997

H5N1 ~ The Avian Flu

• virus subtype that occurs mainly in birds• highly contagious among birds – can even be

deadly• H5N1 virus does not usually infect people, but

infections with these viruses have occurred in humans.

• Most of these cases have resulted from people having direct or close contact with H5N1-infected poultry or H5N1-contaminated surfaces.

http://images.chron.com/content/news/photos/05/05/08/avian1.jpg

http://ec.europa.eu/health/ph_threats/com/Influenza/images/influenza.jpg

Sources

• http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3591814&dopt=Abstract

• http://www.globalsecurity.org/security/ops/hsc-scen-3_flu-pandemic-deaths.htm

• http://www.stanford.edu/group/virus/uda/index.html • Black, J., 2002 Microbiology: Principles and

Explorations. 5th ed. New Jersey: John Wiley and Sons, Inc., 762 p.

• http://www.cdc.gov/flu/avian/gen-info/facts.htm • http://www.cdc.gov/flu/avian/gen-info/spread.htm