Early research suggests a quarter of people or more may still be infectious with COVID-19 after seven days, the standard isolation period across Australia.
‘How long is COVID infectious?’
That is the simple question posed in a recently published article in the journal Nature. It includes caveats that the answer is rarely as clear cut as the query and depends on the interaction of several different variables.
It cites new variants, vaccination, different levels of natural immunity, and behavior as factors that can affect how quickly the virus leaves the system.
The thrust of the findings, however, is of ‘mounting’ evidence that a significant minority of people are likely to be beyond seven days, the current infectious period in Australia for those who test positive to the virus.
One pre-print study cited in Nature looked at the duration of viral shedding. The research, which was carried out by US researchers from Massachusetts hospitals and Harvard Medical School, suggests that as many as a quarter of patients – none of whom had severe disease – are still infectious at day eight.
‘In this longitudinal cohort of individuals with symptomatic, non-severe COVID-19 infection, we found no difference in viral kinetics between omicron variant infection and delta variant infection or by prior vaccination history,’ the authors write.
‘Over 50% of individuals had replication competent, culturable virus at day five, and 25% had culturable virus at day eight.’
For infectious diseases physician and microbiologist Associate Professor Paul Griffin from the University of Queensland, the question surrounding infectiousness is a useful one. He would like to see people have greater awareness of the potential to spread SARS-CoV-2 even after isolation periods end.
‘It is important for people to understand that while on pragmatic grounds we isolate people for what is now a relatively short period of time, that certainly doesn’t mean [they] are no longer infectious when they exit that isolation,’ he told newsGP.
‘It’s great to have a basic understanding of how long the average person is likely to be infectious for.
‘It would be really useful to let people know that a significant proportion of [them] are still going to be infectious beyond day seven.’
Associate Professor Griffin says other studies suggest even more patients could be infectious after seven days. He also agrees that an individual’s infectiousness is not a hard-and-fast rule.
‘Part of that comes down to symptoms and while they don’t individuals necessarily correlate absolutely with whether people are infectious or not, we do know that symptomatic do things that are more likely to facilitate transmission, like coughing for example,’ he said.
Another pre-print study, published last month, used PCR tests to assess the level of infectiousness in more than 700 people and again suggested a substantial amount of people have sufficiently high viral loads to pass on infection from seven days to 10.
Most states and territories already have good guidance on what to do after emerging from isolation to minimise the chances of spreading the disease, according to Associate Professor Griffin.
‘There’s a lot in most of the states’ recommendations [suggesting] that people should still take additional precautions for the following seven days after they exit isolation,’ he said.
‘That typically includes adherence with mask wearing and avoiding unnecessary visits to high-risk venues, like hospitals and nursing homes, for example.’
Other respiratory illnesses
Associate Professor Griffin believes the infectiousness of COVID-19 is likely to be ‘fairly similar’ to other respiratory viruses but says there are still gaps in evidence.
‘We probably don’t have as great data on a number of them, because there’s not been the same level of intensity of study,’ he said.
‘Even the common cold virus is probably similar, in that it’s likely to be [infectious] beyond a week for a proportion of people.’
Another newer potentially complicating factor affecting infectiousness could be the wider use of oral COVID-19 treatments.
Recently a high-profile example was given by the US President Joe Biden, who had the virus, tested negative, then shortly afterwards tested positive again.
The White House doctor said it was a ‘rebound’ case seen in a small percentage of patients who take the oral COVID-19 treatment nirmatrelvir plus ritonavir (Paxlovid), according to Reuters.
Amy Barczak, an infectious-disease specialist at Massachusetts General Hospital in Boston, told Nature that the drugs are likely to have an effect.
‘Antivirals change the dynamics of symptoms, change the dynamics of the immune response and change the dynamics of how you shed,’ she said.
‘I think this is really important, because people are out in the world thinking they’re not infectious after 10 days. But if they have Paxlovid rebound they might be.’
Associate Professor Griffin makes a final point about vaccination.
‘It’s something that’s very hard to study,’ he said.
‘While there may not be great evidence for this at the moment, it’s still highly plausible that vaccinated individuals are likely to be infectious for a shorter period as well.
‘We need better data to reinforce that, but that’s very likely to be an impact of being vaccinated and something that people should also take into account.’
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