It accompanies perhaps 10 to 30 per cent of Covid-19 infections and includes a worryingly large range of lingering symptoms, from debilitating fatigue and aches to brain fog and depression. Yet there's still no test, treatment or cure for Long Covid – nor is there any clearly-identifiable cause. What are scientists learning about it? Science reporter Jamie Morton looks at three of the latest findings.
It's more commonly reported in women
Long Covid affects everyone – including people who have a comparably mild illness with Covid-19 – but studies are increasingly indicating some are more at risk than others.
In New Zealand, Māori and Pasifika populations who've been disproportionately affected by Covid-19 are expected to be among the hardest hit here.
This month, a major US analysis that drew on 35 studies and data from 1.3 million patients found women 22 per cent more likely to suffer Long Covid – and with substantially different symptoms to men.
Women were more likely to suffer from a variety of symptoms, including ear, nose, and throat issues, mood, neurological, skin, gastrointestinal and rheumatological disorders, as well as fatigue.
Male patients, however, were more likely to experience endocrine disorders such as diabetes and kidney disorders.
The study authors, from Johnson and Johnson, said differences in immune system function between females and males could be an important driver in the differing symptoms they found.
"Females mount more rapid and robust innate and adaptive immune responses, which can protect them from initial infection and severity," they said.
"However, this same difference can render females more vulnerable to prolonged autoimmune-related diseases."
Other factors might also play a role: for instance, women may be at greater risk of exposure to the virus in certain professions, such as nursing and education.
Further, they said, "there may be disparities in access to care based on gender that could affect the natural history of the disease, leading to more complications and sequela."
Vaccination matters - but breakthroughs happen
Being vaccinated against Covid-19 remains one of the most important steps people can take to avoid hospitalisation or death with the virus.
One large study recently published in the BMJ suggested that vaccines may also help lessen Long Covid symptoms in some people, but more data was needed before any firm conclusions could be drawn.
The researchers first drew on data from more than 28,000 UK adults who'd received at least one vaccine dose after testing positive with Covid-19, and then tracked their symptoms over seven months last year.
Long Covid symptoms were reported by about a quarter of those people at least once during follow-up and before vaccination, the odds of experiencing these changed little over time.
The researchers found a first vaccine dose was associated with an initial 13 per cent drop in the odds of Long Covid, although it was unclear whether that improvement was sustained over 12 weeks before a second shot was given.
After the second jab, they noted a further 9 per cent decrease - and this improvement held up over at least nine weeks of follow-up.
Unfortunately, however, vaccinated people were still at risk of Long Covid, as shown by one troubling study out of the US, where 7.5 per cent of the population may already be living with the condition.
That research, carried out among a sample of 13 million people by scientists from the Washington University School of Medicine in St Louis and the Veterans Affairs St Louis Health Care System, indicated vaccination against the virus cut the risk of Long Covid by only about 15 per cent.
Yet vaccines were also shown to be most effective in preventing some of the most worrisome manifestations of Long Covid – lung and blood-clotting disorders – which declined about 49 per cent and 56 per cent, respectively, among those who were vaccinated.
Reinfection is a Long Covid risk
Another study by the same team – and released online last week ahead of peer review – found reinfection with the virus could pose further problems.
"Our results show that beyond the acute phase, reinfection with Sars-CoV-2 contributes substantial additional risks of all-cause mortality, hospitalisation, and post-acute sequelae in the pulmonary and broad array of pulmonary organ systems," they reported.
Again, the research involved a vast number of patients: 257,427 people with first infection, 38,926 people with reinfection, and 5,396,855 non-infected "controls".
Compared to those with a first infection, those with reinfection had at least one condition linked with Long Covid that still lingered on even six months afterwards.
"Given the likelihood that Sars-CoV-2 will remain a threat for years if not decades, we urgently need to develop public health measures that would be embraced by the public and could be sustainably implemented in the long-term to protect people from re-infection," they said.
"Pharmaceutical interventions to lessen both the risk of reinfection and its adverse health consequences are also urgently needed."
Among the big unknowns still facing scientists were the impact of different variants.
While a UK study published in The Lancet this month indicated Omicron may be less likely than Delta to cause Long Covid, it didn't account for new Omicron subvariants.
As well, the variant's greater ability to spread further and wider inevitably meant more Long Covid cases.
"This study included data from the first omicron variant BA.1, whereas here in New Zealand, our first Omicron wave was largely driven by BA.2 which is substantially different to BA.1," University of Auckland immunologist and Long Covid researcher Dr Anna Brooks said.
"And of course now, BA.4, BA.5 are on the rise. We won't know about the prevalence of Long Covid from our large outbreaks until we start tracking rates formally."
Brooks said these latest studies showed that vaccines alone weren't enough to protect against developing Long Covid and other long-term health complications.
"That is why limiting infections and reinfections using public health measures, especially masks, is still so important, given several Omicron subvariants are already circulating."
With a second Omicron wave likely to soon take off and drive a surge of reinfections, Brooks and other experts have repeatedly hit out at the Government over a lack of care and support for long haulers.
They've also urged the Ministry of Health to consider their suggested plan - dubbed Vaccines Plus - to tackle the Covid-19 threat.
The ministry has gone as far as designing a new framework and pulling together a "toolkit" that health services could use, while also actively monitoring evidence, with literature reviews every six to eight weeks.
A newly-established expert advisory group – including Māori representation and people with lived experience of long Covid – met for the first time last month.