Mao BMC Public Health 2011, 11:522 http://www.biomedcentral.com/1471-2458/11/522 place, and neighbor household (Figure 7). For each strategy, the resultant shares of infections (%) were cal- culated by counting the number of infections occurring at each type of location, and dividing it by the total number of infections during the epidemic. For the base- line scenario (R0 = 1.4), the workplaces and homes had the same share of infections (38%). As the extension days increase up to 3 days, the weekend-extension stra- tegies heavily reduced the share of infections at work- places from 38% to 20%, while greatly elevated the share at homes from 38% to 50%. The shares of infections at neighbor households and service places were also slightly enlarged by 3%. Discussion Analyses above imply that the effectiveness of weekend- extension strategies is sensitive to the length of exten- sions, the compliance level of businesses, and the infec- tivity of influenza virus. The three-day extension strategy is capable of controlling seasonal flu epidemics, and even prevents the epidemics if a high compliance level can be achieved. The reason is that individuals would have far fewer contacts due to a largely reduced weekday schedule. Most infections are limited within households, but cannot spread out to workplaces until individuals go back to work, making the transmission inefficient. However, the weekend-extension strategies alone are not able to control pandemic flu, because this Page 8 of 10 virus strain is so contagious that only a few human con- tacts could sustain the chain of transmission. To be effective in flu pandemics, these strategies need to be complemented by other pharmaceutical strategies that offer direct protection to individuals, such as the mass vaccination and antiviral prophylaxis. Comparison showed that the three discontinuous extension strategies (Table 3) are less effective than their continuous counterparts (Table 2), because the resultant attack rates are 1~3% higher. This suggests that longer and less frequent interruptions on influenza transmission would be more effective in disease control than shorter and more frequent interruptions. A prob- able explanation is that the transmission of influenza would be doubly effective if infectious individuals con- stantly meet with susceptible individuals at both homes and workplaces. A continuous weekend extension allows disease transmission between household members, but eliminates the transmission among co-workers for a relative long period (e.g., 3~5 days). Hence, the possible routes for transmission are quickly exhausted at homes, and epidemics cannot further develop until individuals go back to work. In contrast, a discontinuous weekend extension allows influenza not only spreading within households, but also transmitting intermittently within workplace contexts (e.g., every other day). The pool of susceptible individuals can be replenished at a short time interval, resulting in more infections. Figure 7 Share of Infections by location. Share of infections (in percentage) at the four types of locations respectively, as a result of the six weekend-extension strategies under R0 = 1.4. All percentages are averages of 50 randomized model runs.