Why trust vaccine statistics from the mid-20th century, if ELISA tests were not available and disease diagnoses were completely subjective?
Diagnoses were not completely subjective; there are myriad assays besides ELISA.


Not back then.

https://emedicine.medscape.com/article/966220-workup

Although the diagnosis of measles is usually determined from the classic clinical picture (see Clinical), laboratory identification and confirmation of the diagnosis are necessary for the purposes of public health and outbreak control. Laboratory confirmation is achieved by means of serologic testing for immunoglobulin G (IgG) and M (IgM) antibodies, isolation of the virus, and reverse-transcriptase polymerase chain reaction (RT-PCR) evaluation.

None of this existed in 1970.