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Explainer: Why a person fully vaccinated against Covid-19 can still get infected and what it means

SINGAPORE — News of a migrant worker who contracted Covid-19 even though he had been fully vaccinated may have caused some alarm, but infectious disease experts said that this is an expected development and there is nothing to be too concerned about.

Medical experts said that the strength of the human body's immune response due to the Covid-19 vaccine varies across individuals.

Medical experts said that the strength of the human body's immune response due to the Covid-19 vaccine varies across individuals.

  • A 23-year-old worker in Singapore was found to be infected with Covid-19 despite being vaccinated
  • Medical experts said that such incidents are ‘true for all vaccines’ and is not a cause for concern
  • One expert said the risk of infection is lowered by at least 90 per cent for mRNA vaccines
  • They also said that even if vaccines do not stop an infection, they will likely reduce its severity

SINGAPORE — News of a migrant worker who contracted Covid-19 even though he had been fully vaccinated may have caused some alarm, but infectious disease experts said that this is an expected development and there is nothing to be too concerned about.

On Sunday (April 11), the first such infection here was reported by the Ministry of Health (MOH). The 23-year-old male Indian national had received both required doses of the Covid-19 vaccination on Jan 25 and Feb 17. 

Similar cases around the world have been reported before. 

The United States’ Centers for Disease Control and Prevention (CDC) said in April that of the 2,479 vaccinated people in a study, three had confirmed coronavirus infections after they were fully vaccinated.

In Japan, a hospital worker who had completed the vaccination regimen was confirmed to be infected on April 11. 

In Singapore over the last three months, MOH had recorded at least three separate cases of people who had been tested positive for the virus after their first dose of the vaccine. 

For these three cases, the ministry said that it is possible for an individual to be infected just before or just after vaccination, since it typically takes a few weeks for an individual to build up immunity after taking the shots.

For the latest fully vaccinated case, MOH said this is “a reminder that it is possible for vaccinated individuals to get infected”.

TODAY takes a closer look at the issue of post-vaccination infections for Covid-19.


The chances of someone getting infected by the coronavirus that causes Covid-19 can happen but such incidences are minimal and some breakthrough incidents are to be expected, experts said.

This is because the strength of the immune response due to the vaccine varies across individuals.

Dr Hsu Li Yang from the Saw Swee Hock School of Public Health at the National University of Singapore (NUS) and leader of its infectious diseases programme said: “For those whose immune systems are compromised, and even for those with otherwise normal immune systems, sometimes they are not able to mount an adequate response after exposure to the virus.

“This is true for all vaccines, not just Covid-19 vaccines, so it is in that sense not a cause for concern.” 

Agreeing, Associate Professor Alex Cook, vice-dean of research at the NUS Saw Swee Hock School of Public Health, said that with more than a million people in Singapore vaccinated with at least a first dose, this is the first person here to have been fully vaccinated yet was still infected. 

“To me, that’s not cause for concern. It’s rather a reiteration of just how good these vaccines are,” he said. 


MOH said on Sunday that the infected migrant worker’s vaccination likely accounts for his lack of symptoms. His positive serology test for past infection was also likely because he has produced antibodies following the vaccination. 

Agreeing, the experts said that even if the vaccine does not fully prevent Covid-19, it could prevent more serious cases from occurring. 

Dr Leong Hoe Nam, an infectious diseases specialist at Mount Elizabeth Novena Hospital, said that a vaccine has the ability to “frameshift” the manifestation of the disease. For instance, a potentially severe Covid-19 case could become a moderate case if the person is vaccinated, and a moderate case could become a mild case, and so on. 

“If the person was found to have the infection without symptoms, this is good news because he could have been a moderately severe case with risk of dying and certainly hospitalisation,” he said. 


Experts said that while the Covid-19 spread among vaccinated individuals is still being studied, the likelihood of post-vaccination spreading is likely to be reduced. 

Professor Paul Tambyah, president of the Asia-Pacific Society of Clinical Microbiology and Infection, said that there is presently no evidence anywhere in the world that a vaccinated individual who develops a mild or asymptomatic form of the disease has spread the coronavirus to anyone else. 

“This is similar to cases of rare vaccine failure in people with chicken pox and measles,” he added. 

Dr Hsu added that worldwide studies have shown that individuals that are successfully vaccinated with mRNA vaccines — such as the Pfizer BioNTech and Moderna vaccine — have shown that their risk of infection is lowered by at least 90 per cent. 

“This means that they are unlikely to spread the virus to others,” he said. 

Covid-19 mRNA vaccines contain bits of the virus' genetic material and not the whole virus. They “teach” cells in the body to make a protein, which acts to trigger an immune response against the coronavirus.

For the minority of people who still get infected after vaccination, they may still pass the virus on to non-vaccinated individuals. 

The experts added that this phenomenon is still being studied. 

Dr Cook said: “The (vaccination) trials mostly haven’t been able to show how much the reduction in disease is due to stopping infections (of non-vaccinated individuals) versus stopping disease among those who get infected.”

This is why infectious disease experts are keeping a close watch on countries such as Israel, which has instituted a mass vaccine roll-out where almost 60 per cent of its population is inoculated, in order to make sense of how the infection numbers are curbed there, he added. 


The experts here agreed that at least 70 per cent of the adult population should be inoculated to achieve herd immunity.

In accounting for mutations in the virus, then the safe figure could be as high as 90 per cent. 

Dr Cook said that the early phase of the Covid-19 outbreak in Wuhan, China, suggested that the reproduction rate of the virus could be between two and three, which means that 10 infected people could infect 20 to 30 people before they recover. 

If the reproduction rate is two, 50 per cent of the population needs to be vaccinated with a perfect vaccine to reach herd immunity, and if the reproduction rate is three, then 66 per cent of the adult population would need to be vaccinated to attain herd immunity. 

“Given that the vaccines are not perfect, and there are some people we can’t vaccinate like children, we really should be aiming for as many adults as possible to get vaccinated, ideally 80 to 90 per cent,” he said. 

Dr Leong said that about a 70 to 80 per cent vaccination rate is enough for the current Covid-19 variant, but the efficacy of the vaccine may also be lowered due to variants of the virus from mutations such as the E484K mutant. The efficacy of a vaccine is the measure of how it performs during a controlled clinical trial. 

Such variants have been shown to reduce the effectiveness of the Pfizer BioNTech and Moderna vaccines to about 60 per cent. In such a case, a 90 per cent vaccination rate would be needed to ensure herd immunity, Dr Leong said.


Experts are divided on whether the world is able to revert to the way it was before Covid-19 struck, with some believing that the virus will become endemic — or constantly maintained at a baseline level. 

Dr Hsu said that it will be difficult to fully eradicate the illness, given how easily it spreads, and that the only successfully eradicated disease through vaccination is smallpox. 

“I believe we will adapt to (Covid-19) over the next few years,” he said. “Widespread vaccination will protect the vast majority of people, in much the same way we are protected against diseases like measles, polio and diphtheria.” 

Other experts said that Covid-19 may be eradicated at the domestic level, but countries cannot close borders forever and may continue to let in travellers who could bring with them variants of the Covid-19 virus. 

Dr Cook said that Singapore would be ahead of the curve in vaccinating its population, but other countries in the region may take a year or more to vaccinate their populations.

“This means that there are more chances for the virus to mutate and lessen the protection we get from vaccines,” he added. 

“Having said that, once enough people are vaccinated, the pay-off from continued interventions is much less than it was last year, and I doubt we’d really need to continue the mask-wearing and safe distancing.”

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