2009 H1N1 Influenza -- Just the Facts: Vaccine Essentials

John G. Bartlett, MD


November 23, 2009

In This Article
Editor's Note: This article will be updated frequently, so check back often for new information. On September 30, 2009, updates were added on vaccine supply and vaccine response in children. On October 14, 2009, updates were added on FDA vaccine approval and supply; vaccine types, priorities, dosing, contraindications, and side effects; vaccine interaction and comparative effectiveness; mandates for healthcare workers; influenza vaccine coverage in the United States; and findings of a survey about the public's intent to be vaccinated. On October 28, more information on vaccine safety was added, along with supply information for both H1N1 and seasonal flu vaccines, how to locate flu vaccine clinics, and news about the "antivaccine movement." On November 5, updates were added about vaccination rates in healthcare workers, vaccine supply, and state disparities in Web-based flu vaccine availability information for consumers. On November 16, updates were added about mandatory vaccination of healthcare workers, background disease rates and vaccine safety, results of a survey of people who tried but were unable to get the H1N1 vaccination, and reasons for the shortage of seasonal flu vaccine. On November 23, a VAERS data summary was added, along with information about the second-dose requirement for children under 10 years of age and effectiveness of the seasonal flu vaccine against H1N1 influenza. For the latest information on 2009 H1N1 influenza, please go to the H1N1 Alert Center.


Update on Influenza A (H1N1) 2009 Monovalent Vaccines

The FDA has approved 4 vaccine preparations. The following data highlight relevant issues:

  • All influenza vaccine preparations in the United States for the 2009-2010 season contain residual egg protein and none contain adjuvant;

  • Children 6 months to 9 years of age who are given influenza A (H1N1) monovalent vaccine should receive 2 doses separated by about 4 weeks; persons ≥ 10 years of age should receive 1 dose;

  • The influenza A (H1N1) monovalent vaccines were made according to standards used for seasonal and influenza vaccines and have the same age group indications, precautions, and contraindications as vaccines that are FDA-approved for seasonal flu; preliminary data indicate that the safety and efficacy of the 2009 Influenza A (H1N1) monoclonal vaccine is the same as for seasonal flu vaccines;

  • Side effects, including local pain at the injection site, were reported in 46% of recipients, and systemic reactions (headache, malaise or myalgias) were reported in 45%; the safety profile is consistent with the experience with seasonal flu vaccine;

  • Influenza activity due to influenza A (H1N1) increased in September 2009 and is expected to continue through fall and winter;

  • There is minimal evidence of significant antigenic change since the first characterization of the virus in April 2009, indicating that the virus continues to be well matched with the vaccine strain; and

  • The vaccines of the 4 suppliers have some differences that are important to recognize (Table 1):

Table 1. FDA-Approved Influenza A (H1N1) Vaccines

Supplier Vaccine Form Mercury
µg/0.5 mL
(nasal spray)
Live virus 0.2 mL
(nasal spray sprayer)
0 2-49 yrsb
Sanofi (IM)a Inactivated
0.25 mL prefilled syringe
0.5 mL prefilled syringe
5 mL multidose vial


6-35 mosb
> 36 mosb
> 6 mosb
Novartis (IM)a 5 mL multidose vial
0.5 mL prefilled syringe
< 1.0
≥ 4 yrsb
> 4 yrsb
CSL Biotherapies, Inc (IM)a 0.5 mL prefilled syringe
5.0 mL multidose vial
> 18 yrs
> 18 yrs

a0.5 mL doses contain 15 µg hemagglutinin of the vaccine strain A/California/7/2009 (H1N1)
bTwo doses separated by 4 weeks for children 2-9 years
(CDC. Update on influenza A (H1N1) monovalent vaccines. MMWR Morb Mortal Wkly Rep. 2009;58:1100-1101.)

NIH studies show that the 15-µg dose without adjuvant generates an immune response that is expected to be protective in most 10- to 17-year-olds at 8-10 days post-vaccination. Younger children had the expected reduced response and will require 2 doses separated by at least 21 days. Infants younger than 6 months are too young for influenza vaccines.

Inactivated Intramuscular Vaccine

The inactivated H1N1 vaccine uses killed virus and is intended to prevent 2009 H1N1 (not seasonal flu) and is available in single dose or multidose vials. Some inactivated 2009 H1N1 vaccines contain the preservative thimerosal. (Note that some people have questioned a relationship between thimerosal and autism, but a 2004 review by the Institute of Medicine found no association, and subsequent studies have supported this conclusion. (Immunization Safety Review Committee. Vaccines and Autism. Institute of Medicine. Washington, DC: National Academies Press; 2004. Available at: https://www.nap.edu/catalog.php?record_id=10997 Accessed October 5, 2009.) The H1N1 vaccine will not prevent seasonal flu, and seasonal flu vaccine will not prevent 2009 H1N1 influenza.

Children 6 months to 9 years of age should receive 2 doses separated by 3 weeks. Children 10 years and older and adults should receive 1 dose.

The following groups should receive the vaccine as soon as it becomes available:

  • Pregnant women;

  • People who live with or care for infants younger than 6 months of age;

  • Healthcare workers (HCWs) and emergency medical personnel;

  • Persons 6 months to 24 years of age;

  • Persons 25-64 years of age who have chronic diseases (including immunodeficiency states) that pose risk for influenza.

When more vaccine becomes available, the following persons should be vaccinated:

  • Healthy persons ages 25-64 years; and

  • Adults 65 years of age and older.

Patients on the high-priority list should be vaccinated as soon as the vaccine is available. Individuals with the highest priority, as listed above, should be vaccinated first, but stringency of methods to restrict vaccine use is unclear.

Contraindications to the inactivated vaccine include:

  • Previous Guillain-Barré syndrome;

  • Life-threatening reaction to previous influenza vaccination; and

  • Severe illness (a "moderately or severely ill" patient is advised to wait, but a mild upper respiratory infection or other illness is not a contraindication).

Note that pregnancy and breastfeeding are not contraindications to receiving the inactivated vaccine. The most common side effect with the killed virus vaccine is soreness at the injection site. Concurrent administration with other vaccines is not contraindicated; seasonal influenza vaccine may be given simultaneously with H1N1 influenza vaccine.

Live Attenuated Nasal Spray Vaccine

The second vaccine is a live attenuated influenza (LAIV), which is given intranasally. LAIV does not contain thimerosal. It is produced the same way as the LAIV that is used for seasonal flu. It is expected that the LAIV for H1N1 influenza will be as safe and effective as the LAIV for seasonal flu. This vaccine will not prevent seasonal flu, and the seasonal flu vaccine will not prevent 2009 H1N1 influenza.

The LAIV for H1N1 is delivered by a nasal spray as a single dose. It is FDA- approved for people 2-49 years of age who do not have contraindications. Groups who should receive the LAIV include:

  • Persons 2-24 years of age;

  • Persons 25-49 years of age who live with or care for infants younger than 6 months; and

  • Persons 25-40 years of age who are HCWs or emergency medical personnel.

When more vaccine becomes available it should be offered to healthy persons ages 25-49 years who do not have contraindications to the vaccine.

Contraindications to the LAIV include:

  • Severe allergy to eggs or other vaccine ingredients;

  • Age 2 years or younger or 50 years or older;

  • Immunosuppression;

  • Chronic medical conditions, including diseases of the heart, lung, kidney, or liver; diabetes, asthma, or blood disease;

  • Children younger than 5 years who have asthma or who have had an episode of wheezing in the past year;

  • Nervous system disease that can cause breathing or swallowing problems;

  • Children and adolescents on long-term aspirin treatment;

  • Any person who has close contact with a person with a severe immunodeficiency;

  • Persons with moderate or severe illness should delay use of this vaccine (a mild upper respiratory infection or similar illness is not a contraindication); and

  • Unlike the inactivated intramuscular vaccine, the 2009 H1N1 LAIV vaccine should not be given with seasonal LAIV because the 2 live virus vaccines must be separated by 1 month. It is permissible to give a killed virus vaccine like the 2009 H1N1 IM vaccine or the seasonal flu IM vaccine with a single LAIV.

Risks and side effects of the LAIV:

  • The most common side effect with the live attenuated nasal spray is nasal congestion in all patients, sore throats in adults, and fever in children 2-6 years.

  • Some children 2-17 years have mild reactions, including nasal congestion, headache, myalgias, fever, wheezing, cough, or gastrointestinal symptoms.

  • Some adults 18-49 years have nasal congestion, cough, fatigue, myalgias, sore throat, or headache.

Comparative Efficacy of Live Attenuated Influenza Vaccine and Inactivated (Killed Virus) Influenza Vaccine

A double-blind, placebo-controlled trial comparing the LAIV vaccine and the inactivated influenza virus vaccine was conducted in 2007-2008 when influenza A (H3N2) accounted for 91% of influenza cases and type B accounted for 9%. There were 1952 participants ages 18-49 years assigned to receive the inactivated vaccine (or placebo) by IM injection or the live virus vaccine (or placebo) by nasal spray. Efficacy was based on frequency of an influenza-like illness that was confirmed as influenza A or B by culture or PCR. The results showed that absolute efficacy against influenza A was 72% for the killed virus vaccine (IM injection) vs 29% for live virus vaccine (nasal sniff). The investigators concluded that the inactivated vaccine was superior to the LAIV for preventing Influenza A (H3N2) in adults. (Monto A, Ohmit SE, Petrie JG, et al. Comparative efficacy of inactivated and live attenuated influenza vaccines. N Engl J Med. 2009;361:1260-1267.)

Effectiveness of Seasonal Influenza Vaccine Against H1N1 Influenza

(CDC. Effectiveness of 2008-09 trivalent influenza vaccine against 2009 pandemic influenza A (H1N1) – United States, May-June, 2009 (MMWR Morb Mortal Wkly Rep. 2009;58:1241-1245.)

The purpose of the analysis was to determine whether vaccination for 2008-2009 seasonal flu had any impact on the probability of infection with pandemic 2009 (H1N1). The method was a case-cohort analysis using surveillance data from 8 states for May-June 2009, for cases of laboratory-confirmed pandemic H1N1 influenza infection in adults older than 18 years of age. Seasonal flu vaccine coverage rates for those states were estimated by the Behavioral Risk Factor Surveillance Survey (BRFSS). The results showed that vaccination rate to seasonal flu in patients with 2009 influenza A (H1N1) was 30%, compared with 29% in the population cohort. Vaccination effectiveness against 2009 (H1N1) was estimated at -10%. These results coupled with other studies suggest that seasonal influenza vaccine does not increase or decrease susceptibility to influenza A (H1N1).


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