The participants filled out questionnaires to assess health behaviors and socioeconomic status, gave a blood sample, and were given the standard trivalent flu vaccine.

The new vaccine is bivalent instead of trivalent — that is, it contains vaccine against two polio strains, Type 1 and Type 3.

The global polio eradication initiative, led by the WHO, has planned a worldwide switch from trivalent oral polio vaccines to bivalent ones next month.