( CNN ) -- Both people who died of swine flu in the United States had pre-existing health problems , federal health authorities said Thursday in a report . A student uses disinfectant as she arrives Thursday at the National Technical Institute in Mexico City . The 22-month-old child who died April 27 of the flu , also called H1N1 , had neonatal myasthenia gravis , an autoimmune disease , said the report , which was written by a virus investigation team at the Centers for Disease Control and Prevention and published online in the New England Journal of Medicine . The child -- who was from Mexico and who fell ill while visiting relatives in Texas -- also had a heart defect , problems swallowing and chronic hypoxia , the report said . The 33-year-old Texas woman who died last week was pregnant when she became ill , the article said . She had been diagnosed with asthma , rheumatoid arthritis and psoriasis . The details came in a summary of the 642 confirmed cases of H1N1 infection that were diagnosed between April 15 and May 5 in 41 states among patients ranging in age from 3 months to 81 years . Sixty percent were 18 years or younger and 18 percent had recently traveled to Mexico , the report said . It said that 36 of the 399 patients with confirmed H1N1 infection whose hospitalization status was known had been hospitalized . Of the 22 hospitalized patients for whom data were available , four were younger than 5 . Nine had chronic medical conditions , some of which included Down Syndrome and congenital heart disease . Seven patients said they had traveled to Mexico during the week before onset of illness . Eleven of those 22 hospitalized patients had pneumonia and eight patients were treated in an intensive care unit . Four were placed on ventilators . As of Tuesday , 18 of the 22 patients who had been hospitalized had recovered , it said . In a conference call with reporters , CDC 's Dr. Carolyn Bridges said epidemiologists would be scrutinizing the disease 's spread in South America , where the cool season is set to begin . Flu virus `` prefers lower humidity and lower temperatures for transmission , '' she said . `` We 'll be looking closely to the Southern Hemisphere during their winter to see what happens . That may give us some clues as to what to expect . '' Brazil 's health minister told reporters Thursday that four cases of H1N1 infection have been confirmed in Brazil . Three have been hospitalized and one is at home , said Health Minister Jose Gomes Temporao . One case is in Rio de Janeiro , two are in Sao Paolo and the fourth was in Minas Gerais in southeastern Brazil . Two of the patients had recently been in Mexico City ; the other two had recently been in Florida , he said . Another 21 cases are suspected in Brazil , he said . In Buenos Aires , Health Minister Graciela Ocana confirmed Argentina 's first H1N1 case -- a man who arrived April 25 from Mexico . The man was hospitalized in the capital and released after he recovered , Ocana said . The ultimate course of the disease remains unclear , said Dr. Anthony Fauci , director of the National Institute of Allergy and Infectious Diseases , who is involved in the effort to create a vaccine . `` We may be seeing the tip of the iceberg , '' he told CNN . `` We ca n't make any definitive projections about where this is going . It appears to be acting like a typical seasonal flu , only it 's out of season . We should n't be seeing this much influenza and it 's with a new virus and that 's the cause fo the concern . '' Asked when a vaccine might be available , he said , `` We hope to get doses by mid- to late fall . '' Journalist Fabiana Frayssinet in Rio de Janeiro and Brian Byrnes in Buenos Aires contributed to this story .
Pre - existing conditions contributed to deaths of U.S. victims , report said . The WHO reports 642 confirmed cases of H1N1 infection . Most hospitalized patients have recovered , report says . WHO reports four cases of pneumonia .