in six countries who participated in a clinical trial that pfizer and biontech began last july half of the volunteers got the vaccine while the other half received a placebo both groups received two shots spaced three weeks apartthe researchers compared the number of people in each group who developed symptoms of covid19 which was then confirmed by a pcr virus test when the companies announced their first batch of results the vaccine showed an efficacy against symptomatic covid19 of 95 percentin other words the risk of getting sick was reduced by 95 percent in the group that got the vaccine compared with the group that got the placebo that result the first for any covid19 vaccine brought an exhilarating dose of hope to the world in december when it was riding what had been the biggest wave of the pandemicsince then the pfizerbiontech vaccine has made up the majority of shots that americans have received with more than 191 million doses given so far according to the centers for disease controlin the new study the researchers followed the volunteers for six months after vaccination up to march 13 over the entire period the researchers estimated the vaccines efficacy was 915 percent against symptomatic covid19 the study did not measure the rate of asymptomatic virus infectionsbut within that period efficacy did gradually drop between one week and two months after the second dose the figure was 962 percent in the period from two to four months following vaccination efficacy fell to 901 percent from four months after vaccination to the march cutoff the figure was 837 percentthose figures still describe a remarkably effective vaccine however and may not convince critics that booster shots are widely neededthe study comes on the heels of data from israel suggesting that the pfizerbiontech vaccines protection may be waning there but experts have pushed back against a rush to approving a booster there the data have too many sources of uncertainty they say to make a precise estimate of how much effectiveness has waned for example the deltadriven outbreak hit parts of the country with high vaccination rates first and has been hitting other regions latersuch an analysis is still highly uncertain said doron gazit a physicist at hebrew university who analyzes covid19 trends for the israeli governmentearlier on wednesday pfizer reported that a third dose of its vaccine significantly increases blood levels of antibodies against several versions of the virus including the delta variantresults were similar for antibodies produced against the original virus and the beta variant which was first identified in south africa pfizer and biontech expect to publish more definitive research in the coming weeksthe announcement was a preliminary snapshot of data contained in an earnings statement and although antibody levels are an important measure of immunity they are not the only metric the body has other defenses that turn back infectionpfizer also said in its statement that vaccines for children ages 5 through 11 years could be available as early as