Amount of virus: When an individual is initially infected with SARS-CoV-2 (day 0), a minute amount of the virus starts replicating inside them until it becomes so abundant that it cannot be contained. The infected individual thus becomes infectious and can spread the virus to others. This transition to becoming infectious usually occurs around day 3. By about the fifth day, infected individuals usually reach peak contagiousness. The person’s immune systems start containing the situation, and from day 5 onward, the amount of virus starts declining. After nearly a week of contagiousness, by about day 9, the virus is repressed. Viral particles continue to remain for weeks or months but are no longer a threat to others.

PCR is better test. It can potentially detect virus very slightly earlier, and for much longer. However, what is important is detecting contagious virus.

1. In order to detect contagious virus, antigen tests appear to work justas well and do not detect virus that does not pose a threat.
2. One may consider two screening strategies: once-weekly PCR or thrice-weekly antigen testing.
3. Lets imagine day 0 is Sunday. If a student gets infected at a weekend playdate, the virus starts replicating, but the level is lesser than the threshold for detection if a PCR test is done on Wednesday. If the test results are received on Friday, they will be negative. This is true for how the student wason Wednesday; but false for Friday.
4. The infectious student has a false reassurance; so do the classmates, teachers, parents, etc. The student stays in school. When the infected student is again tested on Wednesday, the results on Friday show it as positive. This time, it is true for Wednesday but false for Friday (considering contagiousness). After a week of being contagious in school, the student is now sent home to self-isolate and the classroom is quarantined.
5. Now, due to false positives, another classroom ends up shutting down. The teacher had a distant infection from which she has recovered and is no longer a threat. But considering several classrooms now with positive tests, the school has an outbreak and shuts down.
6. If the infected student had been assessed with thrice a week antigen testing, it would have been identified on Friday. The results would have come out immediately, say within 15 minutes. The student would have been advised to self-isolate at home, and close contacts would have been sent home to quarantine. The PCR-positive teacher would have tested negative with an antigen test. The class would not have had to quarantine, and there would not have been a impression of outbreak at the school.
7. Daily antigen screening would be even better. If everyone is screened every day, positive cases can be detected (and isolated) before anyone gets exposed. But even in the absence of daily screening, antigen testing several times a week would allow early detection.

(Medscape)

Dr KK Aggarwal

President CMAAO, HCFI and Past National President IMA