New York, Nov 28 : Covid-19 caused an alarming surge in premature births, but vaccines were key to returning the early birth rate to pre-pandemic levels, according to a new analysis of birth records in the US. During the first two years of the pandemic, maternal Covid infection increases the probability of preterm delivery, defined as a birth that occurs before 37 weeks of gestation, by 1.2 percentage points (from 7.1 to 8.3 per cent). Covid virus caused immune and inflammation responses, and endangered pregnancies via deterioration of the placenta 'This effect is roughly equivalent to in utero exposure to a 9 percentage point increase in the area-level unemployment rate -- or to high-intensity wildfire smoke for 20 days -- an enormous impact,' said the study published in the journal Proceedings of the National Academy of Sciences. Maternal Covid infection also led to higher rates of premature delivery before 32 weeks of gestation, resulting in infants facing the highest risk of mortality, morbidity, and developmental difficulties later in life. However, by 2022, when Covid-19 vaccines were readily available in the US, this effect disappeared -- suggesting that vaccination against the coronavirus may have prevented thousands of preterm births, said researchers from the universities of Stanford and Wisconsin-Madison. Areas with high vaccination rates saw the harmful effects disappear a year earlier than areas with a slower vaccination uptake, they said in the paper. "The effect of maternal Covid infection from the onset of the pandemic into 2023 is large, increasing the risk of preterm births over that time by 1.2 percentage points," said Jenna Nobles, sociology professor at Wisconsin-Madison. "To move the needle on preterm birth that much is akin to a disastrous environmental exposure, like weeks of breathing intense wildfire smoke." The findings show how vaccination against Covid may have helped a generation of US children avoid the long-term health issues and costs associated with premature delivery. As previous studies have shown, children who are born prematurely are more likely to encounter educational and economic setbacks later in life. 'The effects of Covid-19 on infant health may be among the most enduring legacies of the pandemic,' said Stanford sociologist Florencia Torche. The researchers measured the impact of the pandemic with the help of birth records for California's nearly 40 million people, using information on birth timing and the comparison of sibling births to help control for the pandemic's disparate impacts on different demographic groups. They found the excess risk of preterm birth fell slightly in early 2021 before dropping steeply in 2022, at which point maternal Covid-19 infection in pregnancy caused no excess risk of preterm birth for infants. 'Unfortunately, even if the adverse impact of Covid-19 infection on preterm birth has plummeted to zero, this adverse impact is likely to emerge again as the virus continues to evolve and mutate, and as vaccine-driven immunity wanes,' Torche said. /IANS