Author: Dr Mark Ali MBBS BSc FRCS CTh, Medical Director
Can I get a test so I don't have to quarantine?
Quarantining and testing
Why can’t we just test everyone on arrival? It’s ridiculous we aren’t measuring the temperature of passengers when they arrive in this country!
These are examples of very common statements in response to travel restrictions amidst the covid-19 pandemic. Travelling has become a major issue for millions of individuals who find themselves subject to restrictions and requirements in both country of origin and destination. Furthermore, as we have seen with the suddenly-imposed restrictions on passengers returning from Spain, the rules can change quickly.
So, why can’t we just test on arrival? Why do we need to quarantine and avoid all contact with others for 14 days on return from a non-corridor country? It all comes down to the incubation period of the virus.
The incubation period is the time taken between exposure to an infectious organism and developing symptoms. For covid-19, the incubation period is thought to be up to 14 days, with a median time of 4-5 days from exposure to symptom onset. One study reported that 97.5% of people with covid-19 who develop symptoms will do so within 11.5 days of infection. This is the time it takes for the virus to replicate sufficiently to cause symptomatic infection or asymptomatic/pre-symptomatic infection to high enough levels that the individual will shed the virus and be infectious.
Imagine Mr X who has gone away to a country with a high number of covid-19 cases. Mr X flies back to England. The last time he is at the high exposure is on the plane home from that country. Therefore we start the clock then (although there is still a risk of course that he could catch it on return!).
Say he comes into contact with the virus the day he comes home. He would come off the plane and feel fine. His temperature would be normal. He would test negative on a swab as the virus has not replicated sufficiently to be detected. What if we test him at day 7? He could be tested but we couldn’t give him the “all clear”. The virus may not have replicated to sufficient levels to be detected. Maybe on day 8 or 9 it has reached that critical level to be detected on the swab, No amount of testing will accelerate the endogenous biological process of infection and replication.
We have to wait until 14 days have past (data suggest that the vast majority of individuals will show symptoms or be asymptomatically infected within this timeframe). After this we can be fairly confident that Mr X is unlikely to be infectious.Testing at this point would be reassuring but before then, we cannot rule out him becoming infectious after testing.
Much as we would love to have a way out of quarantining- it is causing havoc for leisure and business travellers alike- we cannot overcome the simple biological processes at play. That said, a negative test after this 14 day period is a scientifically valid demonstration that someone is virus free. And therefore neither infected nor able to infect others.