As more and more people get vaccinated, it might seem logical to think our need for large-scale COVID-19 testing should be decreasing steadily. But vaccinated people are still in the minority, and may be in regular contact with friends, family, and coworkers who are still vulnerable to severe disease. To protect these vulnerable people, it is important to use highly sensitive COVID-19 tests to detect infections and prevent asymptomatic people from unknowingly infecting others around them.
As scientists and public health experts have noted, vaccines are life-saving tools, but we can’t rely on them alone to solve all our coronavirus-related problems. Other tactics, including masks and testing, are still critical. The better we can control the spread, the sooner the pandemic will be over.
Aren’t Vaccines Supposed to Prevent Infection?
Contrary to popular perception, it is actually quite rare for a vaccine to completely prevent people from becoming infected with a disease. This holy-grail level of protection is known as “sterilizing immunity,” and has only been achieved with a handful of vaccines, such as those for measles and smallpox. The much more common outcome is “effective immunity,” which protects people from severe disease and death but leaves them with at least some ability to become infected. Even if they themselves never experience symptoms, they may pass the illness on to others. There are many examples of highly successful vaccines that only provide effective immunity, such as those for seasonal flu, whooping cough, rotavirus and hepatitis B.
Researchers are still studying just how much our current suite of vaccines decrease transmission of the virus. Based on existing data, it seems likely that the current COVID-19 vaccines confer excellent effective immunity, but do not provide complete sterilizing immunity against the SARS-CoV-2 virus.
Because the vaccines are good at reducing symptoms, a vaccinated person who contracts COVID-19 may not know they are infected – but they may still be able to transmit the disease. If there are multiple asymptomatic people in a chain of transmission, it can be particularly difficult to trace cases back to their source. This can result in what is known as “silent spread,” where the virus proliferates within a community because of unwitting, undetected carriers. To minimize silent spread, we need to test asymptomatic people for COVID-19 so they can take appropriate precautionary measures right away if they are found to be carrying the virus.
It’s also worth noting that certain groups may not yet be eligible or may choose not to get vaccinated at this point. For example, of the three vaccines available in the U.S., none are authorized for children under the age of 16. Similarly, pregnant and breast-feeding women may choose to wait as more direct evidence is compiled. Many others remain wary of the vaccine based on personal beliefs, experiences, and/or misinformation.
At the other end of the spectrum, we don’t know how long immunity associated with the vaccine lasts. While there are a lot of variables, it’s fair to say that the U.S. population might never, at any one point in time, be “fully vaccinated”.
How Should Asymptomatic Testing Happen?
Ideally, states and public health departments will continue offering on-demand asymptomatic testing for individuals by appointment or on a walk-in basis. But to prevent silent spread effectively, we also need regular, systematic testing of asymptomatic individuals, especially in settings that pose higher transmission risks. These include workplaces, congregate care settings, healthcare facilities, and schools.
To see how chains of transmission might play out in a workplace, imagine this scenario: Jane is vaccinated, but comes into close contact with an unvaccinated person who is infected with SARS-CoV-2 but doesn’t have symptoms at that time. Jane contracts the virus, but due to effective immunity of her COVID-19 vaccine, she feels fine. However, at work a few days later, she eats lunch with a group of colleagues, including a few unvaccinated people, who as result contract COVID-19. Some later develop symptoms, but others in the group may silently spread the infection further, triggering an outbreak in the workplace.
“The SARS-CoV-2 virus is going to be endemic. Not only that, it will continue to evolve and mutate. Simply vaccinating without knowing how transmission will change in the general population would be not only wasteful, but potentially dangerous, as we may possibly be producing variants from individuals that have been vaccinated and now transmitted to other individuals,” says Manoj Gandhi, MD, Ph.D., Senior Medical Director of Thermo Fisher Scientific’s Genetic Testing Solutions unit. “Asymptomatic transmission can be a significant source of spread throughout the workplace, schools and communities, enabling large outbreaks which appear ‘sudden’ when in fact they may have been building for days or weeks. Frequent, sensitive, wide-scale testing is the answer to managing the asymptomatic population and controlling outbreaks before they happen.”
Generally speaking, higher viral loads are associated with more severe symptoms. The corollary to this would be that asymptomatic cases generally tend to have lower viral loads compared to symptomatic patients. Due to this, testing methods have to be robust and more sensitive in order detect these lower concentrations of virus in asymptomatic subjects and help prevent silent spread of the virus. PCR-based testing is still the most sensitive and reliable method that is widely available. Therefore, workplaces, schools, and other organizations should partner with laboratories and providers that can provide reliable PCR testing. Some testing partners can deliver results within 24 hours or less, allowing you to take action quickly when infected individuals are identified.
Regular asymptomatic testing won’t always be a fixture of our lives, but for now, it’s an important part of keeping our communities safe. Learn about how routine asymptomatic testing is enabling colleges and universities to bring students and staff back.