A future with Covid-19: When does a virus become endemic?

People at the Covid-19 vaccination site at Hong Kah North Community Club in March.
People at the Covid-19 vaccination site at Hong Kah North Community Club in March.ST PHOTO: LIM YAOHUI

SINGAPORE - When Prime Minister Lee Hsien Loong said on Monday (May 31) that Covid-19 would become endemic, it rounded off weeks of his Cabinet colleagues painting similar scenarios of a virus here to stay.

Finance Minister Lawrence Wong, who co-chairs the multi-ministry task force tackling the pandemic, had suggested no fewer than three times in the past month that the coronavirus was "never going to go away". Education Minister Chan Chun Sing said in mid-May that Singapore must be prepared to live with Covid-19. And Foreign Minister Vivian Balakrishnan said that Covid-19 would be "permanent" and that waves of infections would be a normal occurrence in coming years.

Since at least late last year, public health experts the world over have warned of Covid-19 becoming endemic.

Some tell Insight that with the pandemic still in full swing globally, it would be too soon to brand Covid-19 as endemic anywhere.

Still, there are cues to be taken from the Sars and H1N1 outbreaks, as well as diseases now endemic here such as dengue and influenza.

Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases, defines an endemic disease as one that has settled in a certain population in a geographical region, where the frequency or occurrence persists.

An endemic is two rungs down from the current global situation of a pandemic, with Sars-CoV-2, the virus behind Covid-19, still spreading and affecting large swathes of people worldwide at an unprecedented scale and speed, she says. As at the start of this week, over 3.5 million people had died, out of more than 170 million confirmed cases since Covid-19 was first reported in December 2019.

Between pandemic and endemic levels lies the epidemic, which refers to a sudden, unexpected jump in cases of a disease.

Endemic conditions

Prof Leo says that if Singapore cannot completely eliminate Sars-CoV-2 from circulation and cases pop up in the community at a relatively stable frequency, a state of endemic would have been reached. But she does not deem it necessary to declare this at a definite time and adds: "It is a continuous epidemiological monitoring of the disease frequency."

Associate Professor Alex Cook at the National University of Singapore's Saw Swee Hock School of Public Health agrees. Labelling Covid-19 endemic "isn't a big deal", he says, since most epidemiologists already think that is the state the world will end up in.

In a January survey by science journal Nature, almost 90 per cent of 100 specialists felt Covid-19 would continue to circulate in the global population for years to come. "Even if some countries have been able to crush (Covid-19), like China or New Zealand, others gave up trying early on, so the horses have well and truly bolted," Prof Cook adds.

Life in China has returned to near normal since mid-2020, though Beijing continues to quash sporadic outbreaks.

New Zealand, often lauded for its Covid-19 response, last week had to temporarily suspend travel with Australia after a fresh outbreak in Victoria.

Singapore, too, had to re-impose restrictions from May 16 to June 13 after a spike in local cases. The country has reported over 62,000 cases and 33 fatalities as at Tuesday.

Prof Cook thinks it may be better instead to talk of a "post-pandemic" era - which PM Lee had predicted, in a March interview with BBC, would arrive in three to five years.

"At some point, the pandemic will have ended, but it's very unlikely that will be because the virus has disappeared," says Prof Cook. "Rather, enough people will have been infected or vaccinated such that the virus spreads slower and affects fewer people."

Experts have suggested that 70 per cent to 90 per cent of the population should be vaccinated to achieve immunity. As at Monday, nearly 40 per cent of Singapore's population had received at least one jab since the inoculation programme was rolled out in December last year.

Professor Paul Tambyah, president of the Asia-Pacific Society of Clinical Microbiology and Infection, adds that a likely scenario leading up to endemic status is for the number of serious illnesses and hospitalisations to decline, prompting the World Health Organisation (WHO) to declare the pandemic over. The global body did the same in 2010 for H1N1.

Prof Tambyah sees the virus slowing down its rate of mutation, to become less virulent though more transmissible. "Stable equilibrium will be achieved with generally mild illness for most people and severe disease in the elderly and immunocompromised," he says. "It could be argued that this has started to happen already for Covid-19."

Still, Dr Oliver Morgan, director of the WHO's department of health emergency information and risk assessment, cautions it would be premature to consider any change in Covid-19's current status as a pandemic.

"The virus continues to evolve and we don't fully know whether there'll be a mutation that changes immunity to the circulating variants," he tells Insight, adding that large pools of global populations remain susceptible to infection.

"A lot of different things could happen… This dynamic state might take quite a long time to settle down into a less dynamic state."

Prof Leo says: "We do not know enough about this virus to accurately predict the trajectory." The virus has behaved in an elusive manner, she adds, and the multitude of variants and rapid immune evasion do not give comfort.

Viral lessons

When Covid-19 does settle, one popular view is that it will go the way of everyday illnesses that people have learnt to live with. Prof Leo cites how dengue is present all year round, with surges of cases - epidemics - now and then. It is likely the same will happen with an endemic Sars-CoV-2, she says.

Other researchers believe it will follow the same path as seasonal influenza, or flu, which infects millions and kills about 650,000 people worldwide every year.

In Singapore, the flu circulates year-round; most recent available figures indicate about 600 related deaths occur annually.

Almost all modern flu strains are descendants of a 1918 virus which originated in birds and killed over 50 million people then - making it the deadliest pandemic to date.

Like Sars-CoV-2, the flu virus mutates and there are several lineages, says Prof Cook. "About once per generation, a very different strain of influenza emerges against which humanity has no (initial) immunity - as with Covid-19.

"The last example of that was H1N1 in 2009. That also spread like wildfire around the world."

H1N1 produced over one billion cases and killed more than 280,000 globally. In Singapore, about 430,000 were infected and 21 deaths reported.

Professor Laurent Renia from Nanyang Technological University's Lee Kong Chian School of Medicine says it was discovered retrospectively that H1N1 was like a "normal" flu with most people having a mild disease, even though the virus was very transmissible.

For an example of a virus that is no longer endemic, look no further than the original Sars strain and its outbreak from 2002 to 2004, says Prof Cook. It began in China, and infected 238 people and killed 33 people here, before being wiped out globally.

Prof Renia says that with Sars, most infected people developed symptoms faster so isolating them from the moment of diagnosis helped prevent transmission. Infection control measures thus helped contain and eventually eradicate the virus, which left a global count of 8,000 infections and 774 deaths in its wake.

"With Sars, there were 'only' thousands of cases, not too many to stop us getting the horses back in the stable," says Prof Cook. "It does not look as if Sars-CoV-2 will go the way of its sister, alas."

Prof Leo says: "The virus has already altered our behaviour significantly and disrupted social norms. The question is: How do we sustain the altered behaviours and rejig the social norms, in order to reach a balance with the virus?"


5 endemic diseases in Singapore

1. Influenza (flu) 

Infections occur all year round, with peaks in the middle, end and beginning of the year. In 2009, an outbreak caused by the H1N1 strain was reported in Mexico. Singapore’s first case was a returning student from New York. The World Health Organisation declared it the first influenza pandemic of the 21st century. 

• It is transmitted mainly via respiratory droplets, and contact with nasal or throat secretions.

• It is not the same as a common cold, which is characterised by a runny or stuffy nose. Flu has additional symptoms like fever, chills, headache, fatigue, muscle pain, cough and sore throat, and can lead to serious illnesses like pneumonia. 

• Routine, once-yearly vaccination is recommended for all aged six months and older.

2. Dengue 

Despite sustained efforts to keep the mosquito population small, transmission occurs year round, with periodic outbreaks – typically during hotter months. Last year saw Singapore’s worst outbreak, with a record 35,315 people infected and 32 dead. 

• It is spread by infected female Aedes mosquitoes of the aegypti and albopictus species.

• It causes nausea, vomiting, fatigue, aches, pain behind the eyes or in muscles and joints and, in more serious cases, bleeding in the gums, nose or internally. About one in five diagnosed ends up in hospital. But about 75 per cent of the infected do not show symptoms, enabling further spread. 

• Vaccination is not recommended for those without prior infection as it leads to a higher risk of severe dengue. 

3. Hand, foot and mouth disease

A common childhood illness present all year round, with outbreaks in pre-schools. Adults can also catch it. An epidemic from September to October 2000 led to 3,790 cases, and pre-schools closed for a fortnight. Four died that year, and three in 2001. 

• It is spread by contact with nasal discharge, saliva, faeces and fluids from the blisters of an infected person.

• Up to 80 per cent of infections are asymptomatic. Otherwise, fever, sore throat, rashes, blisters and mouth ulcers are common symptoms. 

• No vaccines are available. Good personal hygiene is key to controlling an outbreak.

4. Tuberculosis 

Up to 30 per cent of older age groups have latent TB. They do not have symptoms and are not infectious, but about 10 per cent of them may later develop active TB. In January, there were two separate clusters involving 18 people who visited Singapore Pools’ Bedok Betting Centre.

• Highly contagious, it spreads through air droplets from coughs and sneezes – but only after prolonged exposure. It cannot be passed through objects or surfaces. 

• Active TB symptoms include a persistent cough for at least three weeks, low-grade fever, night sweats, fatigue, weight loss and chest pain. 

• BCG (Bacillus Calmette-Guerin) vaccination at birth can prevent severe forms of childhood TB infection, but protection diminishes into the adult years. It can be treated with different drugs over six to nine months.

5. Hepatitis A, B, C and E 

New cases do not occur often but, in 2015, a hepatitis C outbreak at Singapore General Hospital affected 25 patients and an independent review committee linked it to seven of eight deaths.

• A and E are transmitted by contaminated food and water, and B and C through blood and other body fluids.

• Hepatitis is an inflammation of the liver that can be fatal. B and C, together, are the most common cause of liver cirrhosis, cancer and related deaths.

• Hepatitis B vaccination is routinely given to all newborns, healthcare personnel and other population groups at risk.

Sources: National Centre for Infectious Diseases, Ministry of Health, World Health Organisation