Epidemic of false positive cases of COVID-19 in the Magdalene Islands

According to Dr. Yves Bonnier-Viger, director of public health at Gaspésie-Îles-de-la-Madeleine, there has been no real COVID-19 infection on the islands in the past two weeks.

Honestly, there were no real ones on the islands for a long time., – the doctor specifies.

However, last week public health reported one case on Sunday, two on Thursday, and another on Monday, April 19. At the end of the previous week, there were also four cases in the archipelago. They are all false results.

These cases, which turn out to be false positives, are curiously good news, as they indirectly indicate low circulation of the virus in the Magdalene Islands and in parts of the Gaspe River.

Imperfect tests

This phenomenon of false positive test results is not new.

“All screening tests have their limits and are imperfect,” recalls Dr. Bonnier-Wieger.

Sometimes, recovered patients still carry viral particles for several months after they recover. The test result is then positive, but these people no longer get sick.

Tests also have characteristics, including their sensitivity, that is, their ability to detect positive cases when they exist, and their specificity, which ensures that they are indeed negative cases.

For the COVID-19 test, specificity is more important than sensitivity. In other words, the test can be more accurate (98.5% to 99.9%) if the test is negative.

An employee of a laboratory specializing in nucleic acid testing for COVID-19.

Photo: Associated Press.

On the other hand, there is always a chance that the test will identify something as a virus that is not there.

This is a test, therefore the prevalence of the disease in this group is of great importance. In groups where the virus has little spread and where we test a lot, test problems are more obvious, says Director of Public Health Gaspe.

However, screening must be offered to a population with a sufficient prevalence of infection for the test we need to remain valid.

Quote from:Yves Bonnier-Viger and Ariane Courville, Excerpt from COVID-19 Screening Strategy, July 2020.

Also for this reason, random testing has never been the strategy of choice.

In the Madeleine Archipelago, Dr. Bonnier-Viger points out that false positives do arise from inherent test problems.

This phenomenon occurs in the islands, but may also become more and more common throughout Quebec when most of the population is vaccinated. On the Magdalene Islands, this share is almost 80%.

Thus, false positives are less common in Gasp, where the virus is still circulating, including the UK variant, which is more contagious.

Until now, the doctor specifies that in Gasp, the majority of people who have received a positive test result are people who came from another region, as well as their relatives. The epidemiological situation is favorable enough that we do not worry about maintaining the yellow level at the moment., comments by Dr. Yves Bonnier-Viger.

Test is not a diagnosis

Positive results must be confirmed either by epidemiological studies or by a second test. These are observation data, not diagnostic data, the health director warns.

Despite everything, the number of false positives in the archipelago is surprising, and this process is being studied. We find it high too and we search with them [les microbiologistes]… In fact, it should be slightly less.

In Gasp, laboratory research is carried out by the Rimouski-based Optilab services, and on the islands, under the supervision of the Quebec laboratories. We are in contact with these two groups of microbiologists.– says Dr. Bonnier-Viger.

Active cases versus test results

These false positives are part of the regional daily COVID-19 case report, suggesting more infection than it actually is.

The balance is adjusted on the following days. All cases are always re-tested. When it is a truly active case, it remains active for at least 10 days, while incident cases that are not true [il reste actif] 24 or 48 hours later– says Yves Bonnier-Viger.

It is important to note that these false positives remain on the official summary report, according to which about fifty people have become infected on the islands since March 2020. This is a distorted portrait.

To get a more accurate picture of the epidemiological situation in the archipelago, as well as in other parts of Gaspe, a more accurate picture is provided by the active case curve. This number is zero in the archipelago and has been declining in recent weeks in Gasp.

With information from Isabelle Laroz

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