By Dr. Bonnie Henry - Provincial Health Officer

VICTORIA - Testing for COVID-19 and other infectious diseases is a core tenet of public health.

It tells us the what, who and where of disease transmission, but we need to understand the limitations of testing to do this right.

Many have asked and many continue to ask about who is getting tested for COVID-19 in B.C. and why we don't just "test, test, test everyone." What I can tell you is we adapted our testing approach as we learned more about the virus and the test, and as more tests became available, and we will continue to adapt as we progress through our pandemic response.

Scientists at the BC Centre for Disease Control were some of the first in the world to develop a COVID-19 test - the "what" in our testing strategy. This test detects the genetic material of the virus in a sample (usually a swab taken from the back of the nose) from someone who is sick. It is used across Canada and around the world to determine if someone is infected.

Initially, B.C.'s testing strategy was based on identifying the source of transmission to understand who was getting and spreading the virus to guide our response.

That's why we first focused on returning travellers, in combination with leveraging our annual active influenza surveillance testing. This allowed us to put in place the necessary precautions, orders and restrictions we have today.

As the pandemic progressed in B.C. and we had evidence of community transmission, we adapted our testing strategy to focus on those most vulnerable to serious illness from COVID-19.

We did this because we knew the "where" of transmission was now potentially "everywhere." We also knew that many people with mild illness don't need tests and can safely self-isolate and recover at home.

So, we concentrated on the sickest patients, health-care workers, those in long-term care homes and those connected to an outbreak. This gave us an understanding of the impact of COVID-19 on hospitals and care homes.

Of course, testing is not the only public health strategy. Step in step with testing is the contact tracing that our public health teams always do and will continue to do as we transition to Phase 2 in BC's Restart Plan. This is vital to setting up a firewall around every case and breaking the chains of transmission.

As our rate of new cases slowed into mid-April, we adapted our strategy again to broader community testing. This means anyone with symptoms of COVID-19 can now be tested.

It's still important to remember not everybody needs a test and the tests we have are not perfect. If someone tests negative, it may mean they are too early in the illness for the test to detect it yet - a false negative.

When we have a small number of people who actually have COVID-19 in the population, the number of false positives can also be very high. That means we are telling people they are infected and maybe immune to COVID-19 when they aren't.

What's next? Soon we will introduce a "serology" test that can tell us if someone has been infected with COVID-19 in the past and has developed antibodies to the virus. Once the serology test has been validated for accuracy, the test results will be used to get a better sense of where the virus has been and how to further prevent its spread as we move into the next phase of our pandemic.

Understanding the limitations of the tests, our strategy has never been about just testing everyone, but rather using the tests we have to understand who is or has been infected, so we can best protect our families, health-care system and communities.

Our strategy is working, even as we develop it further to meet our evolving needs. It is adapted to our pandemic experience that is helping all of us to hold the line and get through this storm. It will continue to support us as we move through the next phases of this challenging pandemic.