The Power of III
However, this fall there have been major outbreaks in school districts in or adjacent to Buffalo, raising concerns of re-infections. Such re-infections are supported by anecdotal reports of people infected in the spring and fall, or twice in the fall.
The recently released data on the clinical trial in Australia suggests that such re-infections may be linked to low titers generated during mild infections. Although prior pandemic H1N1 infections were among exclusion criteria, 31% of enrollees had titers of 40 or more at baseline. Since there is little cross reactivity between contemporary seasonal H1N1 and pandemic swine H1N1, the baseline titers are almost certainly due to pandemic H1N1 infections that were not disclosed or not know the enrollee. Since many who are infected experience mild disease, including infections without fever, it is likely that many or most of these enrollees had mild infections.
Figure two suggests that although these enrollees had prior antibodies, the titers were relatively low (the vast majority were between 40 and 160) and most had significant improvements (increases of 4 fold or more) after a single injection, suggesting the weak responses to earlier natural infections would provide limited immunological protection, which would support common re-infections. . . . . more at link above