# 3838
Although we’ve heard a great deal over the past few months about the lack of pre-existing immunity to the H1N1 `swine flu’ virus, there is growing evidence that at least some people carry limited immunity to this pandemic virus.
From the start, it was evident that this virus was largely sparing those over the age of 60, which has led many to speculate that a similar influenza A virus may have circulated in the 1940s or 1950s.
Two new studies, published online ahead of print in the CDC’s Journal of Emerging Infectious Diseases, hypothesize that over time exposure to other circulating influenza strains – or perhaps repeated seasonal flu vaccinations – may have helped to provoke a limited immune response in some people to this novel pandemic strain.
Preexisting Immunity to Pandemic (H1N1) 2009
Z. Xing and C.J. Cardona (186 KB, 5 pages)
Serologic Survey of Pandemic (H1N1) 2009 Virus, Guangxi Province, China H. Chen et al. (155 KB, 4 pages)
The first study has received some mainstream media press over the past 24 hours, with a Reuters piece by Julie Steenhuysen widely reprinted around the world.
Study explains immunity to H1N1 in older people
Wed Oct 14, 2009 5:51pm EDT
By Julie Steenhuysen
CHICAGO (Reuters) - Older people who have been infected with or vaccinated against seasonal flu may have a type of immunity produced by cells that protects them from the swine flu virus, U.S. researchers said on Wednesday.
They said the pandemic H1N1 virus has parts found in earlier flu strains, and some people past age 60, who may have been exposed to similar viruses in their youth, may have some latent immune cells that protect them.
"These findings indicate that human populations may have some level of existing immunity to the pandemic H1N1 influenza and may explain why the 2009 H1N1-related symptoms have been generally mild," said Carol Cardona of the University of California Davis School of Veterinary Medicine. Her study appears in the journal Emerging Infectious Diseases.
Cell-mediated immunity, as referenced in this study and article, refers to the non-pathogen-specific immune response mounted by the body when it detects a foreign invader.
The authors hypothesize that repeated exposures to flu viruses (or perhaps repeated vaccinations) help sensitize the immune system to recognize those areas of the flu virus that remain constant across many strains.
This allows some people to mount a limited immune response – which can help fend off or moderate an illness - even against a virus they’ve never specifically encountered before.
The second study looked at a historically unvaccinated population in China and found a very low incidence of neutralizing antibodies to the novel H1N1 virus. A rate far lower than was detected in similar serological studies undertaken in the United States.
This has led the researchers to question whether repeated seasonal flu vaccination – rather than exposure to circulating flu viruses - has led to a higher rate of immunity among American test subjects.
Therefore, we suggest that vaccination against seasonal influenza, rather than exposure to older, seasonal, influenza viruses (H1N1), which may be genetically and antigentically similar to pandemic (H1N1) 2009 virus, as suggested (7), might have generated partial protection against this new virus. No persons in our vaccinated control group had neutralizing antibodies against CA04.
We hypothesize that the absence of neutralizing antibodies in our control group, all of whom had been vaccinated 3 times, suggests that prolonged and repeated vaccination is required for partial immunity to CA04 or that older vaccines may confer some degree of protection. If these serologic differences are indicative of increased susceptibility, we would expect higher infection attack rates in largely unvaccinated populations than in vaccinated populations in countries such as China.
No definitive answers here, but these two studies raise a lot of very interesting questions.
