The source of these figures most likely arrives from a book first published in 1927, which sourced data from a 1920 General Register Office report.
However, it is wrong to say the lifting of quarantine and social distancing measures was the second wave’s cause.
In reality, public health measures greatly varied, with parades still being held in Autumn 1918, at the second wave’s peak.
And quarantine measures were implemented during the second wave in the US.
The precise reason for the flu’s second wave remains a mystery.