The Centers for Disease Control and Prevention is urging Americans to keep wearing masks and social distancing, even as vaccination rates increase.Dr. Rochelle Walensky, director of the CDC, warned a fourth surge could happen if cases continue to rise across the nation.The national average of cases jumped about 10% over the past week, according to the CDC.California appears to be defying the national trend, with cases at 1.8% positivity rate, one of the lowest in the country. California health officials said one in five Californians over the age of 16 is fully vaccinated.It’s unlikely the state will experience a fourth surge, according to doctors.“People are taking this seriously in California and Sacramento,” said Dr. Nicolas Sawyer, an emergency medicine physician with UC Davis Health.Sawyer said continuous efforts to stop the spread of the virus are paying off.“California is doing a good job continuing to mask up, social distance and we’re doing a rather good job of getting people vaccinated,” Sawyer said.As the state’s case rate continues to decline, Sawyer said it’s critical Californians stay vigilant and get vaccinated, when possible.“We have a number of safe and excellent vaccines,” Sawyer said. “As soon as we can get those administered to the majority of the population and reach herd immunity, we're going to be able to return to a sense of new normal.”Starting Thursday, Californians 50 and older will be eligible to get vaccinated.Two weeks later, on April 15, everyone 16 and older will be eligible across the state.