MEXICO CITY, Aug 16 (Reuters) - As the Delta variant of the COVID virus sweeps through Mexico"s cities, more adults in their 30s and 40s are ending up in the hospital with polls showing vaccine hesitancy is rising in younger age groups. At the height of the pandemic in January, 10% of people hospitalized for COVID-19 were aged between 18 and 39, according to the health ministry. Cases have now surged again to near-record levels and that percentage has tripled. "When the virus enters places where there is fertile soil, either because there are fewer people vaccinated or there are more susceptible people," said Alejandro Macías, a Mexican infectologist, "it will finally make those who are not vaccinated sick." There is no major historical anti-vaccine movement in Mexico, unlike the United States and Europe. But the spread of false information about COVID-19 vaccines across the major social media platforms, and from religious groups, appears to be slowing uptake, along with wait-and-see attitudes and a sense of invincibility among the young, experts said. A late July survey by Consulta Mitofsky found 7.2% of people polled said they did not want the vaccine, up from 2.9% in early July. A global study by Facebook (FB.O) and the University of Maryland from late July found as many as 11.3% of Mexicans would choose not to be vaccinated, still far lower than in the United States where almost a third of the population has yet to get a first shot. "Anti-vaccine groups are very harmful," said Laurie Ann Ximénez-Fyvie, lead investigator of the molecular genetics laboratory at the National Autonomous University of Mexico (UNAM). "In Mexico anti-vaxxers have had more influence on the younger age groups." "The (young) people have been more in contact and, for some reason, believe more and follow these groups more." One person who does not want the vaccine is Eduardo Espinola, a 42-year-old nutritionist. "The main reason is because I don"t believe in the vaccine and I do believe in certain side effects that it may have," he said. "It"s even ridiculous to want to sell the idea that the vaccine is going to immunize us. One of the reasons why it does not work is that the strain for which the vaccines were made is one that is no longer in circulation," Espinola added. "It is an unfinished vaccine." Recent data suggest the Pfizer (PFE.N) and Moderna (MRNA.O) vaccines are less effective at preventing infection against the Delta variant than previous versions of the coronavirus, but that they remain highly protective against hospitalization and death. About 40% of Mexico"s 126 million people have so far received at least one vaccine dose, mainly older adults. In Mexico City, home to many younger people, almost a quarter of those aged 30-49 have not turned up for their first shot months after becoming eligible. The government has only just begun to vaccinate people under 30. "The majority of people hospitalized with COVID-19 right now are under 52-years-old and the great majority are people who have not been vaccinated," Mexico"s vaccine tsar Hugo Lopez-Gatell told reporters in late July. Further threatening to sideline Mexico"s vaccine campaign are misinformation and conspiracy theories shared on social media and messaging platforms such as WhatsApp and Telegram, falsely claiming vaccines cause dangerous side effects or contain spyware-like microchips. U.S. anti-vaxxers have claimed that COVID-19 vaccines make people magnetic but there are no microchips in vaccines and most side effects are mild such as sore arms or headaches. Infectious disease expert Susana Lopez from the National Autonomous University of Mexico said the tendency of people to spend more time online had made them more reliant on social media for information. "With social media we are better informed, but we also face a lot of false information and myths," she said, adding that once false information had gone viral it was almost impossible to counteract. "In the Mexican Society of Virology, we are very concerned about the anti-vaccine movement." Reporting by Diego Ore; Additional reporting by Abraham Gonzalez in Mexico City and Julia Symmes Cobb in Bogota; Writing by Cassandra Garrison; Editing by Frank Jack Daniel, Bill Berkrot, Lisa Shumaker and Daniel Wallis Our Standards: The Thomson Reuters Trust Principles.