For many people worldwide, having cotton swabs thrust up their nose or down their throat to test for COVID-19 has become a routine and familiar annoyance.
But two years into the pandemic, health officials in some countries are questioning the merits of repeated, mass testing when it comes to containing infections, particularly considering the billions it costs.
Chief among them is Denmark, which championed one of the world's most prolific COVID testing regimes early on. Lawmakers are now demanding a close study of whether that policy was effective.
Japan avoided large-scale testing and yet weathered the pandemic relatively well, based on infection and death rates. Other countries, including Britain and Spain, have scaled backtesting.
Yet repeated testing of entire cities remains a central part of the "zero-COVID" plan in China, where leaders have threatened action against critics.
The WHO urged countries to "test, test, test" all suspected cases after the coronavirus was first identified. Global surveillance helped scientists understand the risk of severe illness or death, as well as the risk of transmission. Now, with the dominance of the relatively milder Omicron variant and the availability of vaccines and more effective treatments, governments should consider more strategic policies, such as population sampling, experts said. Pulling back too drastically, however, could leave the world blind to a still-changing virus, some officials said.
WHO guidelines have never recommended mass screening of asymptomatic individuals - as is currently happening in China - because of the costs involved and the lack of data on its effectiveness.
Denmark ultimately recorded similar case numbers and death rates as other countries with less widespread testing. This has prompted a majority of parties in parliament to call for an investigation into the strategy.
In the last two years, Denmark's population of 5.8 million logged more than 127 million rapid PCR tests, all provided free. In total, Denmark spent more than 16 billion crowns ($2.36 billion) on testing, according to the Danish Critical Supply Agency.
Neighbouring Norway, with similar population size, only performed 11 million PCR tests, while Sweden, home to nearly twice as many people, completed around 18 million, according to Our World in Data.
Other experts – and the Danish government - said widespread testing reduced the transmission rate and helped people re-enter society, boosting the economy and their own mental health. The economy took a relatively milder hit than other European countries, according to a government report released in September.
One Danish study published last year concluded that the testing program and subsequent isolation of confirmed cases helped reduce transmission by up to 25%.
Other disease experts question such estimates. A review published in Medical Virology in late March on the use of rapid tests for people without symptoms in mass screening initiatives found "uncertainty" over their impact.
There are several possible explanations why testing did not yield a bigger benefit, including an over-ambitious target and the fact that the tests were imperfect. Plus many people either did not or could not isolate after testing positive: a review in the British Medical Journal, pre-Omicron, found that only 42.5% of such cases stayed home for the entire isolation period.
In England, free COVID tests are now only available for government healthcare workers, those with certain health conditions, and people entering hospitals. Others, even with symptoms, have to pay for tests or are simply advised to stay at home until they feel better.
Some global health experts say such a pullback goes too far.