OLYMPIA — This week the Washington State Department of Health (DOH) released the latest statewide situation report on covid-19.

Report findings include:

• Population immunity is helping control transmission, but isn’t enough to counteract risky behavior. The report includes two estimates of the reproductive number (Re), which tells us how many new people each covid-19 case will infect. The behavior-based Re only reflects the impact people’s actions have on transmission, with a best estimate of 1.72 on April 2. The total Re also includes the effects of immunity from vaccination or previous infection, with a best estimate of 1.29 on April 2. To reduce case and hospital admission rates, the total Re needs to stay well below one for a substantial amount of time, and it depends on both population behavior and immunity.

• At the start of April, the best model-based estimate of overall population immunity was 26.8%. At that time, immunity from vaccination (about 15% of the population) was starting to surpass immunity from previous infection (about 11%). These estimates include the time it takes to develop immunity after completing vaccination. They also reflect the fact that vaccines are not 100% protective and a very small number of fully vaccinated people will still get covid-19.

• The estimated percentage of the population with active covid-19 infections almost doubled between March 1 and April 2. This percentage is known as overall prevalence and the estimate includes identified cases as well as people who have covid-19 but don’t have symptoms and may not have been tested. When prevalence is high, it means a lot of people may need health care or could be passing the virus to others. The best model-based estimate of prevalence on April 2 was 0.28% of the population, or just over 21,400 people.

• Statewide case counts and hospital admissions are increasing. As of April 8, case counts were increasing in the majority of counties, including all five of the largest counties, almost all medium-sized counties and many small counties. Other counties are mostly seeing flat trends.

• Case rates are increasing across all ages, except people 70 and older. Data as of April 8 show particularly sharp increases and the highest case counts in people ages 10-49, with shallower increases in children ages 0-9 and adults ages 50-69. Children ages 0-9 consistently had the lowest case rates until mid-March, but now have higher rates than people 70 and older.

• Cases associated with variants of concern (disease strains that may spread faster, cause more severe illness or affect antibodies’ ability to recognize the virus) are increasing. The modeling report estimates that as of April 20, about 50-60% of all cases in the state were likely due to the B.1.1.7 variant and about 30-35% of cases were due to a combination of B.1.429, B.1.427 and P.1 variants. DOH’s latest sequencing and variants report (https://tinyurl.com/WA-covid-variants) shows a 32% statewide increase in positive tests for variants over the past week, with the largest increase detected for the P.1 variant.

“Vaccination is working, but immunity isn’t high enough yet to combat increasing disease levels. All of us, including people who are fully vaccinated, need to keep taking steps to slow the spread while we vaccinate more people,” said Acting State Health Officer Scott Lindquist, MD, MPH. “Keep your social circles small and whenever you plan an activity with others, take it outside. Wear your mask every single time you're around others, indoors or outdoors. If you’ve gotten your vaccine, you still have a role to play – encourage and help people you know to get vaccinated.”

DOH partners with the Institute for Disease Modeling, Fred Hutchinson Cancer Research Center, University of Washington and the Microsoft AI for Health program to develop these reports every other week. More covid-19 data can be found on the DOH data dashboard.

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