Five to 20% of Americans get the flu every year, resulting in an estimated 200,000 hospitalizations and 36,000 deaths. The Centers for Disease Control and Prevention (CDC) recommends the flu vaccine as the single best way to prevent the flu. Vaccines work by exposing immune cells to foreign antigens , provoking them to mount an immune response by creating antibodies .

In early October, just as this year’s flu vaccination season was set to get underway, a major supplier of the flu vaccine reported that none of its 48 million doses would be available to the US, because of contamination during the manufacturing process. Suddenly, the US had lost about half of its vaccine supply, leading to shortages across the country.

Two studies released early and set to be published in the November 25, 2004 New England Journal of Medicine report that lower doses of the flu vaccine, injected into the skin, produce immune responses in healthy adults, aged 18–60, that are equal to full-strength doses delivered by the traditional injection into muscle. The skin injections were not as effective in people older than 60 years. These studies suggest a potential way to stretch this year’s flu vaccine supply.

About the Study

In the first study, researchers randomly assigned 238 study subjects to receive either a standard dose of flu vaccine delivered into the muscle ( intramuscular injection), or a reduced dose (40% of the standard dose) delivered into the skin ( intradermal injection). Approximately 21 days after vaccination, the researchers collected blood samples from the study participants and measured the levels of antibodies present against each strain of influenza included in the vaccine.

Each treatment group was divided into two age brackets: 18–60 years and older than 60 years. For each age group, the researchers compared the antibody responses of the intramuscular vaccination group to the antibody responses of the intradermal injection group.

Anyone taking drugs that suppress the function of the immune system or who were pregnant or breastfeeding were excluded from the study.

The second study compared the antibody responses of study subjects aged 18–40 receiving a standard intramuscular dose of flu vaccine to subjects receiving 20% of the standard dose intradermally.

The Findings

In the first study, among the younger study subjects (18–60 years), there were no significant differences in antibody levels for any of the influenza strains between the intramuscular injection group and the intradermal injection group. Among the older study subjects, intramuscular injection provoked a slightly stronger antibody response than the intradermal injection for all four influenza strains in the vaccine, but this response was only significantly stronger for one of the four strains.

Local pain was significantly more common in 18–60 year-olds who received an intramuscular injection, but local inflammation was significantly more common in participants of all ages who received an intradermal injection.

In the second study, the antibody responses of the intradermal injection group were similar to or better than those of the intramuscular group, for all three strains of influenza in the vaccine.

Though striking, these studies had some serious limitations. First, both studies combined included a total of only 338 people, a relatively small number. Second, blood tests were used to measure immune response, rather than actual cases of the flu.

How Does This Affect You?

These studies demonstrate that for healthy adults aged 18–60, lower-dose, intradermal injections of flu vaccine are just as effective as standard-dose intramuscular vaccines.

These studies may prompt physicians struggling with insufficient vaccine doses to stretch their supply by administering lower, intradermal doses when appropriate. This use, though not officially approved by the US Food and Drug Administration, is perfectly legal. And it might be particularly useful for young, healthy people deemed high-priority by the CDC, such as health care workers or caregivers of very young children.

Aside from these cases, the studies are likely to have only limited implications for the current flu vaccine shortage. Intradermal injections weren’t as effective in people older than 60, and were not tested in the very young or in people with weakened immune systems. As a result, they are not suitable for those most at risk from flu complications.

The CDC recommends that priority for the flu vaccine be given to the following groups this season:

  • Children aged six to 23 months
  • Adults 65 years and older
  • Persons aged 2–64 with underlying chronic medical conditions
  • Women who will be pregnant during the flu season
  • Residents of nursing homes and long-term care facilities
  • Children aged six months to 18 years on chronic aspirin therapy
  • Health-care workers involved in direct patient care
  • Out-of-home caregivers and household contacts of children younger than six months

Healthy adults between the ages of 18 and 64 years who want protection from the flu should ask their physicians about a flu vaccine delivered by nasal spray, which contains weakened live flu virus.