(Reuters) - The following is a roundup of some of the latest scientific studies on the novel coronavirus and efforts to find treatments and vaccines for COVID-19, the illness caused by the virus. Pfizer Inc and BioNTech SE said on Wednesday their two-shot COVID-19 vaccine is safe and effective and produces robust antibody responses in 12 to 15-year-olds. The findings will likely allow use of the vaccine in that group before the next school year, Pfizer Chief Executive Albert Bourla said in a statement. Currently, the vaccine is authorized for use starting at age 16. In the new study, involving 2,260 adolescents aged 12 to 15, there were 18 cases of COVID-19 in volunteers who got a placebo and none in those who received the vaccine, resulting in 100% efficacy in preventing COVID-19, the companies said without releasing a formal report. Side effects were in line with the generally mild ones seen in adults, such as injection-site pain, headaches, fever and fatigue, according to the announcement. In a subset of youngsters, researchers measured the level of virus-neutralizing antibodies a month after the second dose and found it comparable to that in 16- to 25-year-olds in adult trials. Other companies are also testing, or planning to test, their vaccines in children. Moderna Inc is studying its COVID-19 vaccine in adolescents aged 12 to 17 and in children aged six months to 11 years. (reut.rs/31xg6HA) Arthritis drug reduces potency of first vaccine dose The first dose of the COVID-19 vaccines from Pfizer/BioNTech and from AstraZeneca Plc produces only weak immune responses in patients being treated with the widely-used rheumatoid arthritis drug infliximab, researchers have found. Infliximab, sold by Johnson & Johnson under the brand name Remicade and available in biosimilar versions, is also approved to treat a range of other autoimmune disorders including plaque psoriasis and ulcerative colitis. In a study of 865 patients receiving regular infusions of infliximab, researchers observed "poor antibody responses" after a single dose of either of the vaccines, which exposes these patients to a potential increased risk of coronavirus infection, according to a paper posted on Monday on medRxiv ahead of peer review. The responses improved after the second dose, which suggests patients on infliximab should not delay their second shot, the researchers said. "Until patients receive a second vaccine dose, they should consider that they are not protected from SARS-CoV-2 infection and continue to practice enhanced physical distancing and shielding if appropriate," they advised. Even after two doses, they found that a small subset of patients failed to mount an antibody response. The researchers added that they suspect their findings will apply to other drugs in the class known as TNF inhibitors, including Abbvie"s Humira and Amgen"s Enbrel, two of the world"s top selling medicines. "Antibody testing and adapted vaccine schedules should be considered to protect these at-risk patients," the researchers said. (bit.ly/31xCKzC) COVID-19 was third leading cause of U.S. deaths in 2020 COVID-19 was the primary or contributing cause of 378,048 U.S. deaths last year, making it the third leading cause of death in the United States in 2020 after heart disease and cancer, according to data released on Wednesday by the U.S. Centers for Disease Control and Prevention (CDC). In one study, which used preliminary data for 2020, researchers found the overall U.S. mortality rate increased for the first time since 2017, by nearly 16%, with COVID-19 accounting for an increase of 11.3%. The highest overall numbers of deaths occurred during the weeks ending April 11 (78,917) and December 26 (80,656). The COVID-19 death rate was highest among Hispanics, followed by Black non-Hispanics; it was lowest among children aged 5 to 14, and highest among people over age 85. In a second study, researchers found that death certificate data for 2020 collected through February 2021 largely confirm the preliminary data in the other report. The CDC pointed out that limited availability of testing for the coronavirus at the beginning of the pandemic might have resulted in an underestimation of COVID-19-associated deaths. (bit.ly/31yLnK6; bit.ly/3mfrAZR) High-risk U.S. adults may not consistently wear masks Most U.S. adults with chronic medical conditions know they face higher odds of severe COVID-19 but that does not mean they avoid high-risk behaviors, survey data suggest. In late 2020, researchers asked nearly 6,000 participants in the Understanding America Study about their perceived risks from the new coronavirus, and their mask-wearing behavior. They also asked if participants had been diagnosed with a variety of known risk factors - chronic lung disease, kidney disease, heart disease, cancer, autoimmune disorders, diabetes, asthma, high blood pressure, or obesity. All of those conditions, except for high blood pressure and kidney disease, were linked with perceptions of higher risks for COVID-19 complications and death among respondents. Compared to people with none of these medical conditions, those with three or more had engaged in about 10% fewer activities in the past week. But only when visiting a grocery store or pharmacy were participants with three or more medical risk factors more likely to always wear a mask than those with no conditions. During other common activities, including visiting with friends, the majority of adults, including the highly susceptible, did not consistently wear masks, the researchers reported on Wednesday in JAMA Network Open. They "understood their risks were higher, but consistent mask wearing remained low," the researchers said. (bit.ly/3rzVVTT) Open tmsnrt.rs/3c7R3Bl in an external browser for a Reuters graphic on vaccines in development.