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Medical Author:

Charles Patrick Davis, MD, PhD

Charles Patrick Davis, MD, PhD

Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

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Medical Editor:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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Flu vaccine effectiveness is judged by the ability of the vaccine to generate an immune response (measured by a blood test known as a hemagglutination-inhibition assay; protection is deemed effective if assay titer of antibodies reaches 1:40 or greater). This applies to both seasonal and H1N1 vaccines. A titer of 1:40 is considered to be protective 21 days after vaccination. Vaccine efficacy also varies from one person to another; one person's titer could be 1:40 while another person's titer could be 1:80 or greater, even if both were given vaccine from the same batch.

Studies of healthy young adults have shown influenza seasonal vaccine to be 70%-90% effective in preventing illness. In the elderly and those with certain chronic medical conditions such as HIV, the vaccine is often less effective in preventing illness. Studies show the vaccine reduces hospitalization by about 70% and death by about 85% among the elderly who are not in nursing homes. Among nursing-home residents, vaccine can reduce the risk of hospitalization by about 50%, the risk of pneumonia by about 60%, and the risk of death by 75%-80%.

According to the CDC, early studies with H1N1 vaccine showed the injected vaccine to be safe and very effective with immunologic protection developing in healthy people. In the 18-64 age groups, 98% were protected, while in those over 65 years of age, 93% were protected. The fact that H1N1 vaccines are monovalent (only one viral type) and closely matched with most H1N1 viruses may be the reason for the higher effective rates than the trivalent seasonal vaccines. Researchers predicted that two doses (shots) would be needed to get a good immune response, but early studies showed that for most people 10 years of age and older, one dose was effective. In pregnant women, a single dose of the injectable H1N1 vaccine showed protective antibodies in 92% of women after 21 days. However, in children 6 months of age through 9 years of age, two doses of the H1N1 vaccine were recommended by the CDC because early U.S. data suggested that a maximum of 55% of children (36 months of age through 9 years of age) showed protective antibodies 21 days after a single injection, and younger children (6 months of age to 35 months of age) were far less protected (25% protective). In contrast, a study from Australian researchers indicated a single H1N1 vaccine dose is effective for children they studied in the same age range. The CDC notes that children aged 6 months to 19 years of age are at higher risk for complications (for example, pneumonia) than most other age groups and need vaccination; the CDC also notes that children 6 months of age or less are very susceptible to influenza, but there is no approved vaccine for these infants. The CDC recommends that caregivers and household members who care for infants 6 months old or younger be vaccinated to protect these infants by surrounding them with people who are not transmitting the virus. Side effects in children are similar to those in adults and may include soreness or redness and swelling at the injection site, mild transient fever, and malaise or rash; most side effects last one to two days. Rare serious side effects may occur in patients that are allergic to the components found in the vaccines (for example, children allergic to eggs), but even these situations can be managed by pediatric specialists.

Nasal-spray vaccines (both seasonal and H1N1) can provide effective protection to individuals 2-49 years of age. The effectiveness is less in the older population. The adequate administration and the low (0.6%-2.4%) but possible potential for spread to another person makes this vaccine restricted to healthy individuals 2-49 years of age.

Effectiveness and safety of vaccines are currently being checked by the cooperative efforts of the FDA, CDC, and other institutions in the U.S. In December 2009, there was a recall of a batch of pediatric vaccine (800,000 doses from Sanofi Pasteur) that was manufactured with too little virus antigen. The remaining doses were recalled. Fortunately, CDC officials said, "No action (is) necessary by a parent of a child who received this vaccine...(The) only thing they need to do is make sure their child gets two doses if they're in the 6-month to 35-month age group, which is the group impacted by the recall." Monitoring of the vaccines is ongoing.

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