In chapter 51, Aosoi speculates that the zombie infection may not be limited to physical attacks by a zombie. Instead, there must have been a large-scaled, pre-planned aerial attack. With the survivors being temporarily immune to it.
To make better sense of this, Aosoi analogies with influenza (the flu). Noting that influenza is a virus that mutates every-so-often, meaning that a single flu shot will not keep a person immune against influenza forever. If Renya had been infected without outside contact, the zombie infection must have mutated to get around his immunity.
In chapter 57, Miki is most certain that the aerial infection had actually took place. This was after she had explained to Aosoi that back before they had gotten the RV, they had to enter the radio station shelter through the rooftop, which was covered in a lot of dust.