- Pregnant women,
- household contacts of infants younger than 6 months,
- health care and emergency services personnel,
- individuals between 6 months and 24 years, and
- those aged 25 years or older with underlying conditions
- that put them at high risk of complications from influenza should be at the front of the line when a vaccine for the currently circulating novel H1N1 influenza virus becomes available, according to advisors to the US Centers for Disease Control and Prevention (CDC).
The Advisory Committee on Immunization Practices (ACIP) in late July, reviewed historical data on pandemic influenza to establish which groups should be considered the highest priorities for vaccination. The CDC is expected to adopt the recommendations as part of their larger efforts to prepare for the upcoming influenza season.
The CDC is currently awaiting more data on clinical trials of the safety and efficacy of H1N1 vaccines under development, including in pregnant women.
Because such trials will provide only partial information, it will be important to monitor for rare adverse events, such as the severe cases of Guillain-Barré syndrome that were associated with the 1976 swine flu vaccinations.
"If there are problems, we want to be able to interpret them quickly, figure out whether they are coincidences or are real, and take action," Dr Anne Schuchat said.
The CDC anticipates 120 million doses will be available at the end of October, followed by an additional 80 million doses 1 month later. However, some manufacturers have expressed some caution, that H1N1 virus grown in eggs has been multiplying more slowly than vaccine developers had hoped.
The priority groups include about 160 million persons, and 2 doses of the vaccine per person are likely to be needed. Vaccine uptake is unlikely to be 100% in these groups—in fact, vaccination for seasonal influenza is recommended for some 280 million individuals but less than half are immunized each year. If uptake is similar for the new vaccines, there may be enough vaccine for those who want it, Schuchat said.
If supplies are not sufficient, the ACIP recommended excluding some individuals from the vaccinate-first priority group, specifically individuals aged 5 through 18 years (unless they have other risk factors),
Historically, pregnant women have had higher rates of complications when infected with novel or seasonal influenza strains, and emerging data suggest the H1N1 strain is no exception.
In a recent publication, scientists noted that pregnant women have been admitted to the hospital with H1N1 infections at a much higher rate than in the general population, 0.32 per 100 000 vs 0.076 per 100 000 (Jamieson DJ et al. Lancet. 2009;374:451-458). Between April 15 and May 18, there were 34 confirmed cases of H1N1 influenza in pregnant women in the United States, and about one-third of these individuals were admitted to a hospital; between April 15 and June 16, of the 45 deaths of US individuals attributed to the virus, 6 were among pregnant women.
Yet despite the risks influenza pose during pregnancy, pregnant women often do not receive antiviral drugs or seasonal influenza vaccination. Only half of US pregnant women with confirmed cases had received oseltamivir, which is recommended by the CDC. Only about 15% of pregnant women are vaccinated for seasonal influenza.
"It's vital for pregnant women who have respiratory illness and fever to get early treatment, and we think antiviral medications for them can be life saving," Schuchat said.
Young adults are another group of particular concern. Older adults, who may have some immunity from exposure to a similar virus in the 1940s and 1950s, have been largely spared from infection with the currently circulating H1N1 virus. Only 1% of confirmed cases have occurred in individuals aged 65 years or older and that the age group of 50- to 64-year-olds has also experienced very few infections. Older individuals should still receive the seasonal influenza vaccine.
Living situations and social behaviors of college-aged individuals aged 19 to 24 years may place them at greater risk of infection. In fact, the flu season may begin earlier than usual this year if H1N1 cases spike when children and college students return to schools, said Schaffner. The H1N1 virus has continued to cause outbreaks at summer camps and summer college sessions.
"We may have a double-barreled flu season: first H1N1, then seasonal influenza," Schaffner said. In anticipation of these increased demands, he recommended that physicians begin providing their patients with the seasonal influenza vaccine as soon as it becomes available in late August or early September.