Patients infected with the Delta strain of Covid-19 are likely to develop moderate or severe symptoms, and mild or symptomless cases of the disease are becoming rarer, a senior Russian scientist said Sunday. "In case of the Delta variant of the infection, the latent period is shorter as the virus has become more contagious. Patients are now developing moderate and severe forms of the disease more often, while mild and asymptomatic ones are getting rarer," said Alexander Gorelov, deputy director of the Russian sanitary watchdog's Central Research Institute of Epidemiology. "The treatment of such patients requires more time and effort, and this affects the overall statistics, leading to a conclusion that the disease lasts longer," he said. Read: 'Different levels of antibodies needed for protection from Covid strains' "The basic reproduction number – showing contagiousness, or how many persons on average can become infected by one carrier before the diagnosis is made – has multiplied by 2-2.5 times. It used to be 1-2, but now it is reaching 5-6," the expert added. "Only mass vaccination can break this chain of transmissions and help us prevent the emergence of new variants of the infection that can be even more dangerous," Alexander said. Read: Knowing your Covid symptoms can help to make informed decisions The Delta variant of Covid-19 was first detected in India last October. Earlier, the All India Institute of Medical Sciences reported that this version of the virus is capable of infecting people even after inoculation with the Covaxin and Covishield vaccines used in India. The Delta Plus version of this strain is considered to be even more contagious.