The Zika virus has been around for at least 70 years, and hasn’t caused any long-term ill effects—and yet the outbreak of microcephaly cases in Brazil coincides with the start of the new vaccination programme, designed to slow the progress of pertussis, or whooping cough.

Pregnant women started to be given the vaccine at the beginning of 2015, and cases of microcephaly have been reported since last October—10 months later. By December, 2,400 cases had been reported, and the Brazilian government declared a state of emergency.

The virus, which is spread by mosquitoes, has been linked to microcephaly because it was found in a baby who had died after contracting the condition. The virus was also found in the amniotic fluid of two mothers whose babies were born with microcephaly.

But activists argue the virus can’t have suddenly started to cause microcephaly when it hasn’t done so for 70 years. In addition to the time coincidence, they say the version of the Tdap vaccine introduced to Brazil was never properly tested, and contains chemicals and preservatives that can affect neurological development.