Timing of local vaccination campaign versus serum collection in our study.

<p>(A) Among 104 healthcare workers who reported receipt of pandemic (H1N1) 2009 vaccine, the proportion of healthcare workers with antibody titer ≥1∶40 to pandemic (H1N1) 2009 by hemagglutination inhibition (HI) or viral microneutralization (VN) by week of specimen collection with 95% confidence intervals; (B) the number of pandemic (H1N1) 2009 vaccines administered to 150,000 healthcare workers in Hong Kong from December 2009 through April 2010 (source: Department of Health, Hong Kong <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0027169#pone.0027169-Department1" target="_blank">[14]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0027169#pone.0027169-Department2" target="_blank">[15]</a>).