I have no idea why you think that there were no studies in April. A quick search shows that's wrong. There were initial reports in March, followed by many peer-reviewed studies that started coming out in April, which were then followed by studies with more objective measures (rather than retrospective self-reports) and larger sample sizes.

[1] https://academic.oup.com/cid/article/71/15/889/5811989

[2] https://link.springer.com/article/10.1007/s00405-020-05965-1

[3] https://onlinelibrary.wiley.com/doi/full/10.1002/alr.22592

[4] https://jamanetwork.com/journals/jama/fullarticle/2765183

[5] https://www.nature.com/articles/s41591-020-0916-2