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H1N1 (Swine Flu): Previous Infections May Help Fight H1N1 Virus & Vaccine Safety

November 22, 2009

IMMUNE SYSTEM OF HEALTHY ADULTS MAY BE BETTER PREPARED THAN EXPECTED TO FIGHT 2009 H1N1 INFLUENZA VIRUS.DOC

A new study shows that molecular similarities exist between the 2009 H1N1 influenza virus and other strains of seasonal H1N1 virus that have been circulating in the population since 1988. These results suggest that healthy adults may have a level of protective immune memory that can blunt the severity of infection caused by the 2009 H1N1 influenza virus.

The study team was led by Bjoern Peters, Ph.D., and Alessandro Sette, Ph.D., of La Jolla Institute for Allergy and Immunology, Calif., grantees of the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health.

The investigators looked at molecular structures known to be recognized by the immune system—called epitopes—on 2009 H1N1 influenza and seasonal H1N1 viruses. Viral epitopes are recognized by immune cells called B and T cells: B cells make antibodies that can bind to viruses, blocking infection, and T cells help to eliminate virus-infected cells.

The study results suggest that healthy individuals may have immune memory that recognizes the 2009 H1N1 strain and therefore can mount some measure of an immune attack. The findings also may help explain why the 2009 H1N1 influenza pandemic affects young children more severely than it does healthy older adults and also why two H1N1 vaccinations are needed to protect children ages nine years and under.

J Greenbaum et al. Pre-existing immunity against swine-origin H1N1 influenza viruses in the general human populace. Proceedings of National Academy of Sciences. DOI: 10.1073/PNAS.0911580106.

http://www3.niaid.nih.gov/NIAID/Templates/Specialized/PreseedableLeftRightNav.aspx?NRMODE=Published&NRNODEGUID={90062EA1-3194-4116-9F92-6356F73D6961}&NRORIGINALURL=%2fnews%2fnewsreleases%2f2009%2fH1N1protection.htm&NRCACHEHINT=Guest#

SAFETY OF PANDEMIC VACCINES

As of November 19, 2009, WHO has received vaccination information from 16 of around 40 countries conducting national H1N1 pandemic vaccine campaigns. Based on that information, WHO estimates that around 80 million doses of pandemic vaccine have been distributed and around 65 million people have been vaccinated.

National immunization campaigns began in Australia and the People’s Republic of China in late September.

Vaccination campaigns currently under way to protect populations from pandemic influenza are among the largest in the history of several countries, and numbers are growing daily. Given this scale of vaccine administration, at least some rare adverse reactions, not detectable during even large clinical trials, could occur, underscoring the need for rigorous monitoring of safety. Results to date are encouraging.

Common side effects

As anticipated, side effects commonly reported include swelling, redness, or pain at the injection site, which usually resolves spontaneously a short time after vaccination.  Fever, headache, fatigue, and muscle aches, occurring shortly after vaccine administration, have also been reported, though with less frequency. These symptoms also resolve spontaneously, usually within 48 hours. In addition, a variety of allergic reactions has been observed. The frequency of these reactions is well within the expected range.

Guillain-Barre syndrome

To date, fewer than ten suspected cases of Guillain-Barre syndrome have been reported in people who have received vaccine. These numbers are in line with normal background rates of this illness, as reported in a recent study Nonetheless, all such cases are being investigated to determine whether these are randomly occurring events or if they might be associated with vaccination.

WHO has received no reports of fatal outcomes among suspected or confirmed cases of Guillain-Barre syndrome detected since vaccination campaigns began. All cases have recovered. WHO recommends continued active monitoring for Guillain-Barre syndrome.

Investigations of deaths

A small number of deaths have occurred in people who have been vaccinated. All such deaths, reported to WHO, have been promptly investigated. Although some investigations are ongoing, results of completed investigations reported to WHO have ruled out a direct link to pandemic vaccine as the cause of death.

In China, for example, where more than 11 million doses of pandemic vaccine have been administered, health authorities have informed WHO of 15 cases of severe side effects and two deaths that occurred following vaccination. Thorough investigation of these deaths, including a review of autopsy results, determined that underlying medical conditions were the cause of death, and not the vaccine.

Safety profile of different vaccines

Campaigns are using nonadjuvanted inactivated vaccines, adjuvanted inactivated vaccines, and live attenuated vaccines. No differences in the safety profile of severe adverse events among different vaccines have been detected to date. Although intense monitoring of vaccine safety continues, all data compiled to date indicate that pandemic vaccines match the excellent safety profile of seasonal influenza vaccines, which have been used for more than 60 years.

http://www.who.int/csr/disease/swineflu/notes/briefing_20091119/en/index.html

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