SINGAPORE - Only a small number of vaccinated Covid-19 cases in Singapore have developed "long Covid" symptoms so far.
This is supported by overseas data showing the risk of developing the condition could be reduced by half in vaccinated people.
The long Covid syndrome refers to residual symptoms that people continue to experience long after recovering from the disease.
These include fatigue, shortness of breath, chest pain, joint aches and "brain fog" - where one's thinking is sluggish - which tend to last for four weeks or more.
However, the risk of developing long Covid could be reduced by half among vaccinated individuals compared with the unvaccinated, according to a study conducted in Britain, said Dr Barnaby Young, head of National Centre for Infectious Diseases (NCID)'s Singapore Infectious Disease Clinical Research Network.
"This is an encouraging result, and I suspect that it may even be an underestimate of the effectiveness of vaccination against long Covid symptoms, which are a direct result of a severe illness," he said.
While there certainly have been cases of long Covid among vaccinated people overseas, the number of cases locally so far appears to be small, said Dr Young.
He noted that locally, there has been a lower incidence rate of "debilitating symptoms because of long Covid", including breathlessness on exertion, dizziness and fatigue, which can be a severe consequence of pneumonia that can take several months to recover.
This is because severe infection is less common in those who were infected post-vaccination, so vaccinated people are protected against these longer-term complications, he added.
However, this is something that needs careful study to determine if it continues to remain true over time, and whether with increased awareness of long Covid, more cases may emerge, noted Dr Young.
In contrast, one in 10 recovered Covid-19 patients, who were unvaccinated, had persistent symptoms six months after their initial infection, an earlier NCID study had found.
Similarly, Professor Paul Tambyah, senior consultant at the National University Hospital (NUH) Division of Infectious Diseases, said there have been no cases of long Covid among vaccinated patients at NUH to date, though it may be too early to tell.
However, Dr Young said the driver of persistent inflammation after a Covid-19 infection still remains unclear, with different explanations behind the cause of the disease for different people.
On the one hand, there may be an autoimmune component, where one may have an overactive immune response, which causes persistent side effects due to the body attacking itself.
However, the persistent inflammation may also be due to fragments of the virus still remaining in the body, or echoes of the initial inflammatory response to infection, noted Dr Young.
Agreeing, Prof Tambyah said that while the mechanism of long Covid is not yet known, perhaps vaccination might reduce the viral load after the fifth day of infection, and thus reduce the antigenic stimulation (antigens that are capable of stimulating an immune response), which triggers autoimmunity or other kinds of reactions.
He added that the long-term symptoms have also been seen in other infectious diseases, such as influenza.
For example, an Oxford University study that compared long-term symptoms of those who were diagnosed with Covid-19 and those who had the flu found that at least 42 per cent of those with Covid-19 had at least one long-term symptom, compared with 30 per cent among the flu group.
Dr Young said that post-viral syndromes are complex and poorly understood.
"This is well demonstrated by the continuing debate and controversy about chronic fatigue syndrome in long Covid. Because of this, I hope that the increasing attention on long Covid will offer insights into other post-viral syndromes," he added.