It feels like it's been a lot longer, but the first case of COVID-19 was officially recorded in December 2019. Researchers are continuing to investigate the full effects of this disease, including unusual ones, and a new analysis has now linked the infection to a rare neurological condition.

Acute transverse myelitis (ATM) – an inflammation of the spinal cord which can cause pain, paralysis and sensory problems – was identified in 43 adult COVID-19 cases across 21 countries, with patient ages ranging from 21 to 73, and also 3 children aged 3 to 14 years old.

The review collects together previous research and case reports, and according to the team behind it, the data are enough to warrant further investigation. In any given year, the incidence of ATM is estimated to be just 1.34 to 4.6 cases per million people.

By contrast, during a 10-month period, the incidence of ATM amongst COVID-19 patients alone has ended up being around 0.5 cases per million, setting off red flags for the researchers.

"We found ATM to be an unexpectedly frequent neurological complication of COVID-19," write the researchers. "Most cases (68 percent) had a latency of 10 days to 6 weeks that may indicate post-infectious neurological complications mediated by the host's response to the virus."

The new research adds to what we already know regarding COVID-19 and neurological complications: the disease has been associated with numerous nervous system issues, such as a lingering 'brain fog' effect.

In these 43 cases, spinal cord lesions were found to lead to quadriplegia and paraplegia, with other associated problems including a loss of bladder control. The research was undertaken after a case was discovered in Panama.

Further cases were then collected from scientific literature published between March 2020 to January 2021.

"This review confirms that ATM is not uncommon as a neurological complication associated with COVID-19 infection around the world, responsible perhaps for 1.2 percent of all neurological complications caused by this coronavirus," the team concludes.

Since ATM is known to be an immune-mediated condition (meaning the primary cause is unclear, but there's an involvement of our immune system and inflammatory processes in the body), the researchers indicate there are some potential immune mechanisms that could explain how SARS-CoV-2 may lead to ATM.

Furthermore, the team also notes that three ATM cases occurred in AstraZeneca vaccine trials. While each was investigated, the researchers note in this study that they might provide a clue as to the immune mechanisms involved.

"The pathogenesis of ATM remains unknown, but it is conceivable that SARS-CoV-2 antigens – perhaps also present in the AZD1222 COVID-19 vaccine or its chimpanzee adenovirus adjuvant – may induce immune mechanisms leading to the myelitis," they write.

Further studies should help find more answers and the antigens involved, but it's another reminder that we're still a long way from understanding everything that COVID-19 brings with it, even as vaccines roll out across the world.

Previous research has identified complications with pregnancies for people who contract COVID-19, and there's also the ongoing problem of long COVID – those who suffer with symptoms for many months after largely getting through the initial illness.

While the coronavirus might be coming under control in more countries in the world – despite the threat of new strains – research into the effects of COVID-19 are going to carry on for a long time after the pandemic is over.

The research has been published in Frontiers in Immunology.