That statement is straight from the paper: "A history of prior vaccination
is not always associated with immunity nor with the presence of specific antibodies".
Sekla L, Stackiw W, Eibisch G, Johnson I. Cadham Provincial Laboratory, Winnipeg, Manitoba. An evaluation of measles serodiagnosis during an outbreak in a vaccinated community. Clin Invest Med. 1988 Aug;11(4):304-9.
During an epidemic of measles in a vaccinated community, five serodiagnostic tests were performed on 67 persons on whom clinical and epidemiological data were available. The test found most suitable for a rapid diagnosis of measles infection was an Enzyme Linked Immuno Sorbent Assay for the detection of specific IgM antibodies. Only one false negative IgM was recorded. In a group of 45 persons who fulfilled the clinical definition of measles, specific IgM antibodies were detected in the acute phase serum of only 30 (66.6%), of whom 17 were vaccinated. When the convalescent sera were tested, specific IgM antibodies were detected in 25 of the 28 (89.2%) vaccinated, and in 17 of the 17 (100%) non vaccinated clinical cases. A convalescent blood should be tested in persons with a rash illness and no IgM antibodies in the acute phase serum. There were individual variations in the time of appearance of IgM. On the day of onset of rash, IgM antibodies were detected in 7 of the 12 (58.3%). A history of prior vaccination is not always associated with immunity nor with the presence of specific antibodies.
It is known that people with NO antibodies may not get a disease and people with high levels of antibody will still get the disease. In other words, the mere presence of antibodies does not equate to immunity.