Measles virus-specific antibody levels in Sudanese infants: a prospective study using filter-paper blood samples

Abstract

We conducted a prospective birth cohort study in rural Sudan to assess measles virus (MV)-specific antibody levels at different time points in infancy. Dried blood spots were collected on filter paper at birth (cord blood) and at ages 6, 12 and 24 months (heel-prick). Maternally derived MV-specific antibody levels were high in cord blood samples, but at the age of 6 months had dropped below cut-off values in half of the infants. By extrapolation it was concluded that the current Expanded Programme of Immunization (EPI) target age for measles vaccination of 9 months was an appropriate choice for this area. At the age of 24 months acquired MV-specific antibodies were detected in 65-85% of the cohort, which corresponded well with the 79% of infants reported to be vaccinated by this age. This study demonstrates the usefulness of filter paper blood samples for seroepidemiological studies in developing countries.

Fig. 1

Results of the four different serological assays used when testing a serial dilution of the 2nd International Standard for Measles [serum 66/202, NIBSC, Hertfordshire, UK, 5 international units (IU) per ml]. (a) ELISA plates coated with an inactivated whole measles virus preparation (–•–) or a recombinant-baculovirus produced nucleoprotein (–▾–) were incubated with the serial dilution of the reference serum at a further dilution of 1:300, similar to the test samples in these assays. After washing the plates were incubated with a peroxidase-labelled anti-human IgG conjugate. Results are shown as the extinction measured at 450 nm (mean±standard deviation of five measurements). (b) F- or H-expressing transfected human melanoma cells were incubated with a serial dilution of the reference serum at a further dilution of 1:100, similar to the test samples in these assays. After washing the cells were incubated with a FITC-labelled anti-human IgG conjugate, and immunofluorescence was quantified in a FACS. Results are shown as fluorescence signal in arbitrary fluorescence units (mean±standard deviation of triplicate measurements).

Fig. 2

Specific IgG levels in cord blood samples (left two panels) or heel-prick samples prospectively collected at ages 6, 12 or 24 months (right six panels). Samples were reconstituted from dried blood spots collected on filter paper, and tested in MV-specific or N-specific ELISA (upper panels, results expressed in IU/ml) or in F- or H-specific FACS-measured immunofluorescence assays (lower panels, results expressed in arbitrary fluorescence units, AFU). In the upper panels the dashed lines indicate antibody levels associated with protection from disease (0·2 IU/ml), in the lower panels they indicate a previously established arbitrary cut-off level (30 AFU). Symbols are shown in black if subjects were vaccinated ⩾1 month prior to sample collection; triangle-shaped symbols are used for subjects who were diagnosed with clinical measles ⩾1 month prior to sample collection. Numbers in the corners of the panels indicate the percentage samples in the respective quadrant.