People who test negative for Covid-19 seven days after coming into contact with a confirmed case are unlikely to be infectious, a new modelling study suggests.
The research, if verified, means that the self-isolation period for virus contacts could potentially be reduced to seven days without the risk of onwards transmission if combined with testing.
The study also indicates that daily testing of contacts for at least 5 days could replace the quarantine period altogether, but the authors stress findings would need to be backed up by real-world tests before being adopted as policy.
Current government guidelines stipulate anyone who comes into contact with a person who has tested positive for Covid-19 must quarantine for 10 days.
That was reduced from 14 days in December last year amid concerns people were not following the two-week rule.
The team of researchers from the London School of Hygiene & Tropical Medicine (LSHTM) is warning people should continue to follow government rules until their research can be further verified.
“Adherence to quarantine rules is key for reducing onward Covid-19 transmission,”said assistant Professor Sam Clifford, joint-lead author and member of the CMMID Covid-19 working group at the LSHTM.
“Our findings suggest that incorporating testing of contacts into a trace-isolate system could potentially help to reduce quarantine times, and this in turn may improve adherence by making it easier to complete the full isolation period.
“Our study did not evaluate costs, however, and further studies are needed to address this, as well as to further verify our findings.”
Assistant Professor Elizabeth Fearon, a collaborator within the CMMID Covid-19 working group at the LSTM added: “Testing on its own will have no impact on transmission if cases are not financially enabled and socially supported to self-isolate after a positive test.”
The study, published in The Lancet Public Health Journal on Wednesday, looked at viral load and the sensitivity of Covid-19 tests.
It used mathematical modelling to estimate the effect of the different quarantine and testing strategies on reducing onward transmission from traced secondary infections.
Using data from Public Health England (PHE) and NHS England, they simulated the levels of virus an infected person would be likely to produce at each stage of infection, alongside timing of symptom onset, test sensitivity and tracing and testing timings.
The model assumes self-isolation adherence levels are moderate, with 67 per cent of people completing the World Health Organisation (WHO) recommended 14-day period after testing positive, and 50 per cent of contacts of confirmed cases completing 14 days.
Based on the assumptions, completing the 14-day period for contacts of Covid-19 cases is estimated to prevent 59 per cent of onward virus transmission.
A similar proportion of onward transmission could potentially be prevented with only seven days of self-isolation if a PCR or LFA test is performed on the final day (PCR – 54 per cent, LFA – 50 per cent), the paper indicates.
Reducing delays, such as in the tracing and notification of contacts, is integral to getting people to quarantine as quickly as possible, the authors say.
The researchers note the model is based on the assumption that the LFA test works at the higher level of sensitivity, detecting on average 76.8 per cent of cases that tested positive in a PCR test.
The analysis of the LFA test using the lowest estimated success rate from the Liverpool mass testing trial, 48.8 per cent, found comparable but slightly lower results.
In this analysis, taking an LFA test on day seven of self-isolation would prevent 44 per cent of onward virus transmission, while daily LFA testing for five days would prevent 43 per cent of transmission, the study suggests.
The authors say their study did not consider other aspects of the test and trace system that might affect virus transmission.
This includes the number of people with Covid-19 who do not engage with the system at all, variation in the number of secondary cases generated by each infected person, and the proportion of secondary cases that are missed by tracers.
Additional reporting by Press Association