Are many women immunized against rubella unnecessarily?

Abstract

Radial haemolysis (RH) was used to test sera for immunity to rubella from 1317 patients attending a general practice. One hundred and forty-one (10.7%) were treated as susceptible and offered an attenuated virus vaccine (RA 27/3). Pre-immunization sera from 43% of these patients were reactive at low levels in RH (less than 15 international units rubella antibody per ml). Pre- (S1) and post- (S2) immunization sera from 66 vaccinees were studied in detail. Antibody was detected by RH, haemagglutination inhibition (HI) and enzyme-linked immunosorbent assay (ELISA), and the specific IgM response was measured by a solid-phase M-antibody capture radioimmunoassay (MACRIA). The vaccine-induced IgM response was only detected if the S1 serum was non-reactive by all tests for rubella antibody. It was weaker than that seen following wild virus infection. It could be detected reliably for six weeks, and in most cases for nine weeks, after immunization. In contrast, patients with S1 specimens reactive by RH, HI or ELISA never showed an IgM response in the S2 specimen despite "significant' antibody rises often being present. It was considered that an IgM response to RA 27/3 was the best indicator of pre-immunization susceptibility to rubella. The failure of many vaccinees to make an IgM response implied that a significant proportion were already immune. It is suggested that the threshold for a report of immunity to rubella could be lowered from 15 i.u. antibody per ml and so fewer women immunized without vaccine being withheld from those who need it.

Rubella antibody screening

“…The results of Mortimer et al (1981) suggest that this could be lowered to 5 i.u./ml. Two questions remain: first, is there proof that such low levels of antibody protect against foetal infection on re-exposure and second, is there any rubella antibody screening test which is suitable for the detection of such low levels of rubella antibody in routine practice ?…”

“…Certainly the results of Mortimer et al (1981) indicate that this can be achieved using RH in a limited, special study. Nevertheless, small zones of 4-6 mm diameter produced by specific antibody are not always easy to recognize around wells 2-3 mm in diameter, particularly if there is some reaction in the control plate.…”

“…Non-antibody inhibitors do not interfere with the RH test at all, and if screening is performed by this method it may not be necessary to use such a high MIT. Mortimer et al (1981) addressed themselves to this problem and asked the question, 'Are many women vaccinated against rubella unnecessarily ?'. They studied 1317 women attending a single general practice.…”

“…If the results of Mortimer et al (1981) are widely applicable then only 60 % of women with less than 15 i.u./ml rubella antibody are truly sero-negative and in need of immunization. Many of the other 40 % could be screened out using the RH test with a lower MIT.…”

Screening for anti-rubella IgMad libitum

“…Both these vaccines evoke an IgM response in the non-immune patient that is easily detectable by the M-antibody capture assay and that persists long enough to be a useful measure of pre-vaccine susceptibility. This may help to decide the management of women inadvertently immunized when they are pregnant (Mortimer et al 1981 b). We expect the new IgM assays to supplant other serological tests for rubella, with the exception of two simple screening tests for antibody, radial haemolysis 1-2 and the haemagglutination-inhibition test.…”