We see a high PFC for MERS for 2 reasons that spring to my mind:
- There are no, or very few, mild cases of MERS. Cases are usually severe and most are being caught. We know there are some mild cases and we know that contacts of confirmed cases are infrequently MERS-CoV positive. I'm assuming contacts are tested with the same rigour as those in hospital due severe MERS (e.g. lower airway sampling with follow-up testing).
- Although we don't see much transmission from cases to contacts we do see some and this may be enough to maintain a small number of community transmission events. The second reason then is that we are only testing cases with severe disease, many of whom die as a result of complications due to the initial infection. Prospective screening of a sample of the well general community and those with "common colds" might identify more cases that would reduce the PFC.