Catalonia makes less tests, but also reduces the percentage of positive results

Santa Coloma de GramenetCatalonia has not yet left the epidemiological danger zone, since it began the lifting of restrictions with fairly high indicators. However, it has managed to bend the infection curve. Despite the fact that fewer tests are done every week than in the past, its positivity rate - the number of tests that are positive in relation to the total number of tests done - is also falling at a good pace: "The fact that both indicators are falling is a sign of peace of mind", says the physicist and researcher from the Computational Biology and Complex Systems Group at the UPC, Clara Prats.

Since July, more than 2.5 million tests have been performed in Catalonia to diagnose new infections. Half of them, between October and November, were done at the height of the second wave. Even so, throughout the last month figures have progressively decreased, and, if on October 21st almost 280,000 tests were being done every week, now 176,500 are being performed - 36% less. The Secretary of Public Health, Josep Maria Argimon, attributes this to the slowdown of the pandemic. "When did we do more tests? Logically, during the second wave, since there were many cases and many infections that had to be detected, which ended up arriving at primary healthcare centres or hospital emergency wards", he explains.

Between November 17th and November 23rd, according to the updated data that avoid the reporting delay, 176,500 tests have been done. On the previous week (10-16 November), 207,500 tests were performed, while 14 days earlier (3-9 November), 257,000 tests were performed. According to the Health Department, this reduction is only due to the fact that the spread of the virus has been delayed and there are fewer infections to detect. In fact, the number of new infections has been falling by an average of 35% each week, despite having antigen tests available for health centres and, at the same time, more laboratory capacity has been released.

Have close contacts stopped being tested, as was speculated about the Community of Madrid, to achieve this data improvement? According to the physicist Clara Prats, all the epidemiological indicators point to the fact that the decrease in cases in Catalonia is real, and not due to fewer tests being performed: "If you diagnose, it is because people with symptoms arrive in the healthcare centres. If the spread decreases, the demand also decreases", she explains. And if not so many symptomatic cases arrive, Prats adds, fewer contacts are explored -according to the Health Department, each person has between four and five close contacts on average- and, therefore, fewer tests are performed.

Only 6% of the tests are positive

The key indicator of whether testing has been discontinued or whether the epidemiological trend is actually favourable is the positivity rate. "When the number of tests drops, and so does the positivity, it is clear that the diagnostic effort has not been reduced. However, if the number of positive tests in relation to the total would increase, it would be because testing was stopped when it shouldn't have been", explains Prados. Catalonia meets the first criterion, and the rate of positivity has been decreasing at a good pace for weeks now.

Currently, 6.4% of the tests that are done are positive, a figure that is close to the threshold (5%) set by the World Health Organization (WHO) to consider that a country has the pandemic under control. Last week it was 8% and fourteen days ago it was 10%. "That the positivity goes down is key", says Argimon.

Now, the more PCRs are done - and therefore the higher the absolute number of PCRs performed - the more cases are being looked for, especially asymptomatic ones, and the better the curve is controlled. That is why the Catalan Health Hepartment has resumed this week the strategy of proactively searching for coronavirus positives through a new batch of mass screening, which will slowly spread throughout the Catalan territory. It does so now that it has managed to bend the curve, and wants to ensure the downward trend of the infection curve while maintaining economic activity.

The other important point of the above strategy is the tracking and follow-up of positives and their contacts. According to the coordinator of the covid-19 monitoring unit, Jacobo Mendioroz, this system has been greatly strengthened since the summer and there are 3,000 people working on it. Currently Catalonia has about 38 trackers per 100,000 inhabitants, when the recommended figure for this population density is of 18 professionals.