Also biased because they are sampling people who are out and about. People who are out and about likely come into contact with more people than people who are staying at home and rarely going out.
Change in Human Social Behavior in Response to a Common Vaccine
https://pubmed.ncbi.nlm.nih.gov/20816312/
Conclusions: These results show that there is an immediate active behavioral response to infection before the expected onset of symptoms or sickness behavior.