2009 H1N1 Influenza -- Just the Facts: Vaccine Essentials

John G. Bartlett, MD


November 23, 2009

In This Article

Vaccine Safety and Adverse Effects

Vaccine Safety

An updated safety report was released by Dr. Bruce Gellin, Director of the National Vaccine Program Office. The report indicates that the government has a "beefed-up" database on safety that shows no signals of concern after the first 10 million doses. One report of a death was caused by swine flu and not the vaccine. (Neergaard L. New group helps US monitor swine flu shot safety. Associated Press, November 1, 2009.)

  • The most reliable source of information about flu vaccine side effects is found in package inserts or ACIP statements

  • Vaccines without adjuvant will be used for 2009 H1N1 and seasonal flu vaccine in the 2009-10 season.

  • Thimerosal is a mercury-based preservative used in some multidose vaccine vials. It is not used in single-dose vials or with LIAV (nasal spray). It is in some but not all multidose injection vials.

  • Only vaccines without adjuvant will be used for 2009 H1N1 and seasonal flu vaccine in the 2009-10 season.

  • Most common side effects are runny nose with the nasal vaccine and pain, redness, or swelling with injected vaccine.

  • Guillain-Barré syndrome: The incidence of Guillain-Barré syndrome, unrelated to vaccines, is about 140 cases per week in the United States. Most cases are associated with recent infections, especially Campylobacter jejuni-associated diarrhea. Most patients with Guillain-Barré syndrome, but not all, will recover. The concern about Guillain-Barré syndrome with flu vaccines is based on the 1976 flu vaccine that was associated with an increased rate of about 1 case per 100,000 vaccine recipients. Most subsequent studies have not found a relationship between Guillain-Barré syndrome and flu vaccines, but 2 studies showed an increase in Guillain-Barré syndrome by about 1 case per 1 million vaccine recipients.

  • Latex allergies: There is no latex in vaccines, but latex may be in the seal stoppers of vaccine vials or in rubber parts of syringes. Patients with latex allergies may have reactions as a result of this exposure.

  • An unpublished report from Canada stated that receipt of seasonal flu vaccine in the 2008-09 season was a risk for developing 2009 H1N1 flu. Preliminary studies in the United States and other countries have not confirmed these results.

Monitoring of vaccine safety will be accomplished with standard methods that include the Vaccine Adverse Event Reporting System (VAERS) and the Vaccine Safety Data Link (VSD).

VAERS data for 2009 HINI influenza vaccine -- through November 9, 2009. (CDC. VAERS. Summary of 2009 monovalent H1N1 influenza vaccine data. Available at: https://vaers.hhs.gov Accessed November 13, 2009.)

Through November 2, 2009, 33.7 million doses of vaccine have been shipped to providers.

  • A total of 1922 adverse events were reported. Of these, 96% were considered not serious. "Serious" is defined as life-threatening, death, permanent disability, abnormal condition at birth, or a reaction requiring hospitalization

  • A total of 84 reactions (40%) were described as serious. These include 6 deaths. These 6 cases are under investigation, but the initial review indicates no clear pattern.

  • Four reported cases of Guillain-Barré syndrome are being reviewed. The expected rate of Guillain-Barré is about 80-160 new cases/week without vaccination

Note that VAERS is a voluntary reporting system established by a CDC/FDA collaboration to detect adverse reactions to vaccines, medications, and other products. This system cannot determine cause and effect, but it may supply signals that prompt analysis of databanks and other resources to get better scientific data. To know the total extent of this surveillance system on vaccine safety, see https://www.flu.gov/professional/federal/monitor_immunization_safety.html

Vaccine Side Effects

Concern about the ability to distinguish common medical events (such as myocardial infarction) from rare side effects of the vaccines has been expressed by US health authorities. Weekly rates of common events (which would occur regardless of mass vaccinations) include 25,000 acute myocardial infarctions, 14,000-19,000 miscarriages, and 300 anaphylactic events. Guillain-Barré syndrome is a rare event routinely noted in 1-2/100,000 population. To detect possible associations of these events with the H1N1 flu vaccine:

  • Using insurance databases for 50 million people, Harvard University will link reasons for post-vaccine physician visits with vaccine administration;

  • Johns Hopkins University will email 100,000 vaccine recipients to ask how they are feeling; and

  • CDC will distribute "take-home cards" that instruct vaccine recipients on how to use VAERS. (Neergaard L. Intense tracking for swine flu shot's side effects. Associated Press. September 28, 2009.)

Background Disease Rates and Vaccine Safety

A recent analysis estimated rates of disease occurrence that would be expected without association with the H1N1vaccine or any other vaccine. Conditions selected were "highly visible" ones that are expected to occur coincident to H1N1 vaccination of 10 million people (Table 6).

Table 6. Predicted Numbers of Coincident, Temporally Associated Events After a Single Dose of a Hypothetical Vaccine, Based Upon Background Incidence Rates

Condition Number Coincident Events Since a Vaccine Dose Baseline Rate Used for Estimates
  1 day 7 days 6 weeks  
Guillain-Barré syndrome (/10 million) 0.5 3.6 22 1.9/100,000 person-years
Optic neuritis (/10 million females) 2.1 14.4 86 7.5/100,000 person-years
Spontaneous abortion (/1 million pregnancies) 397 2780 16,684 Based on UK data -- 12% of pregnancies
Sudden death within 1 hour (/10 million) 0.1 1.0 6 0.5/100,000 person-years

With 10 million vaccinations, a temporal association with 22 cases of Guillain-Barré syndrome and 5.75 cases of sudden death within 6 weeks of vaccination would be anticipated. In female vaccine recipients, there will be 86 cases of optic neuritis per 10 million vaccinated, and 397 women per million vaccinated will have a spontaneous abortion within 1 day of vaccination -- all events that might be expected as background rate events.(Black S, Eskala, J, Siegrist C-A, et al. Importance of background rates of disease in assessment of vaccine safety during mass immunization with pandemic H1N1 influenza vaccines.Lancet. 2009 Oct 31 [Epub ahead of print].)


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