SÃO PAULO — At least 55 pregnant women in Brazil who were infected with the Zika virus have given birth to babies without microcephaly, which the virus usually inflicts upon newborns, a new study has revealed.

Among the children born to these mothers, all from the state of São Paulo, 28% still suffer from some neurological alteration, but the symptoms are much less severe than those detected among babies born of infected mothers in the country's northeast region or in Rio de Janeiro. "We've had none of the very serious symptoms we used to observe," says virologist Maurício Lacerda Nogueira from the Faculty of Medicine of São José do Rio Preto who coordinated study. "This is the first time that we have managed to compare data from two different populations affected by the Zika virus in a reliable way."

There are no doubts about the Zika virus' capacity to affect the development of the brain during pregnancy. Experiments carried out in laboratories and on animals have already demonstrated that the Zika virus has a special "preference" for attacking neural stem cells.

When the Zika epidemic first reached the northeast region in 2015, researchers weren't able to conduct studies on most of the infected pregnant women and were only able to understand the effects that the virus had on babies. The first such results came in 2016, when scientists from the Oswaldo Cruz Foundation in Rio de Janeiro observed a group of 125 pregnant women infected with the virus. In that group, about 40% of the babies were born with serious problems affecting their nervous system, 7.2% died in pregnancy, and four children were born with microcephaly. In comparison, among the São Paulo group of 55 women, not only were neurological issues much less critical, but there were no miscarriages and not a single case of microcephaly.

The obvious question when comparing the data is what could have led to such different results. "There is a co-factor [acting together with the Zika virus], but we don't know which one yet," says study coordinator Lacerda Nogueira. He believes there's very little chance that a genetic difference would explain the higher vulnerability of mothers and babies in the northeast region. Another possibility would be the greater incidence of other infections, such as syphilis or toxoplasmosis, that might have led to fetal malformations among the Northeastern mothers .

Beyond that, scientists don't yet know how previous infections from dengue or Chikungunya virus, both "cousins" of Zika, might weaken or exacerbate the reaction of the mother's body to the new invader. It could also be that the mother's nutrition and general health has an effect on the virus, and that poverty is an aggravating factor.