format specifier of percent sign with Poco Formatter

while using Poco::format I am trying to print the following line: "3% and 5%"

int var1 = 3
int var2 = 5
std::string s;
s = format("%?d%% and %?d%%",var1,var2);

instead of getting "3% and 5%", s equals "3 ?d%"

What am I doing wrong?