News analysis

Vaccinating more people by spacing out doses can slow down Covid-19 spread

Having more people vaccinated should also reduce the spread of the virus in the community.ST PHOTO: ONG WEE JIN

SINGAPORE - With the surge in Covid-19 community cases this month, experts say giving one dose of the vaccine to as many people as possible - instead of two doses to fewer people - is the right thing to do.

Currently, the two doses are given three or four weeks apart.

On Sunday (May 16), Health Minister Ong Ye Kung said Singapore is studying the possibility of giving the second vaccine dose six to eight weeks after the first.

This will allow more people to get at least one vaccine dose.

Singapore is unable to speed up the vaccination process any other way because "the pace is limited by the pace of the supply arriving in Singapore", Mr Ong said.

He also pointed out that older, more vulnerable people have already been vaccinated, and the programme is now moving to younger cohorts.

Professor Ooi Eng Eong, an expert on emerging infectious diseases at the Duke-NUS Medical School, said one dose of the Pfizer or Moderna mRNA vaccine used here gives people 70-90 per cent protection from the 12th day.

The second dose raises the protection against severe illness and death to about 95 per cent.

Having more people vaccinated would not only mean more people being protected, but should also reduce the spread of the virus in the community.

The relatively large number of unlinked community cases - 17 on Sunday and 11 on Monday - is worrying, since it is difficult to stop the spread if it is not known where these people caught the bug from.

Of the 38 community infections reported on Sunday, only 13 had been quarantined prior to detection. It is not known how many people have been infected by the other 25, who were roaming freely in the community.

There could also be many more undetected infected people, who are asymptomatic or only mildly sick, passing the virus on to others.

Giving more people one jab can help reduce such community transmissions.

Prof Ooi said a study in Britain found that transmission to household members by infected healthcare workers, who have had a single dose, was a third lower than among those who were not vaccinated.

It fell to half once both doses had been administered, so it is still important to get two doses.

Explaining why those given one vaccine jab are less likely to spread the disease, Prof Ooi said the immune response could include neutralising some of the virus, and thus lower the infectious fraction that could be transmitted to others.

"It could also lead to shorter period of infection and hence also lower the rate of transmission," he added.

Britain was the first country in the world to space out the first and second doses. It did that towards the end of last year, when Covid-19 was raging through the country.

At that time, many experts condemned the move, fearing it could diminish protective effects of the vaccine.

Scientists also feared that "under-vaccinating" people could result in more variants emerging.


Britain was the first country in the world to space out the first and second doses. PHOTO: AFP

With hindsight, Associate Professor Alex Cook, an expert in infectious disease modelling and statistics at the NUS Saw Swee Hock School of Public Health, said Britain's move "was one of the smartest decisions" and hastened the end of the winter wave there.

"Until now, there hasn't really been a need in Singapore to do likewise. However, the pressure from the rising number of local cases makes it a good point to reconsider that position," he said.

In fact, spacing out the doses longer could actually make the vaccine more effective.

Senior infectious diseases specialist Prof Dale Fisher of the National University Hospital (NUH) said that "there is evidence that waiting even 12 weeks can show an enhanced vaccine response".

Doing some quick back of envelope calculations, Prof Fisher added: "If Singapore expands the interval for those due for second jabs in the next three weeks, it will allow another half a million people to get their first jabs."

As at May 10, more than three million vaccine doses have been administered, with 1.85 million people having at least one dose of vaccine, and 1.28 million people fully vaccinated with both doses.

Prof Fisher does not think people would mind this move, if they know that delaying the second dose does not reduce efficacy, but might actually enhance it. "So it's a very good idea knowing that delaying the second jab is not a problem."

An article in Nature journal last week carried the results of a study by Public Health England of 175 people over the age of 80 years who were given the second Pfizer jab either three weeks or 11-12 weeks after the first.

It found: "The peak antibody levels were 3½ times higher in those who waited 12 weeks for their booster shot than were those in people who waited only three weeks."


Pfizer's Covid-19 vaccine generates antibody responses three-and-a-half times larger in older people when a second dose is delayed to 12 weeks after the first, a British study said. PHOTO: AFP

Associate Professor Hsu Liyang, an infectious diseases expert at the NUS Saw Swee Hock School of Public Health, said: "In places where the virus is spreading rapidly, providing the first dose of the vaccine to more people will prevent severe infections and save many lives.

"This may not be so applicable to Singapore but nonetheless, the downsides are primarily administrative and not clinical or public health."

He added that although there is no data on the effects of postponing the second dose by more than 12 weeks, historical data from other vaccines shows that "a slightly longer delay is likely harmless and will not significantly compromise vaccine efficacy".

The important thing, the experts said, is for everyone who is eligible to get vaccinated.

Prof Cook noted that many of the infected patients in the Tan Tock Seng Hospital cluster had not been vaccinated.

He asked if there would even have been a cluster in the first place had more of the patients been vaccinated - especially the one who likely introduced the virus to the hospital?


Prof Cook noted that many of the infected patients in the Tan Tock Seng Hospital cluster had not been vaccinated. PHOTO: ST FILE

Prof Hsu stressed that getting one dose of vaccine will both protect the one vaccinated, and also reduce the spread of the virus should the person get infected - so it is a good move for both the individual and the nation.

So delaying the second jab in order to have more people getting at least one dose could pull Singapore back from the brink of a second wave of Covid-19 infections in the community.

Associate Professor David Lye, director of Infectious Disease Research and Training had felt impelled to send out a message warning that "our current status is likely more dangerous than just before circuit breaker (CB) last year".

Prof Cook disagrees: "On the day we went into CB last year, we had about 30 people in intensive care units (ICU), and over 1,000 people in hospital or community facilities. Today, we have fewer than 500 people being monitored and two people in ICU."

But he says we should heed Prof Lye's suggestion to take precautions, as we are now facing more virulent viruses.

The four variants of concern, all now found here, are more easily transmitted, so although the numbers we are facing today may not be as big as they were last year, the danger remains very real.

Prof Leo Yee Sin, executive director of the National Centre for Infectious Diseases, said the B1617 Indian variant is not only more transmissible, there seems to be higher viral shedding in respiratory secretion and the time needed for infection to occur appears to be shorter.

She added: "Infection may occur in vaccinated individuals, but they appear to have milder outcome that is not progressing to pneumonia. However we need time to further observe this."

Prof Cook estimates the R0 or transmission rate in Singapore today to be 40 per cent higher than it was just before the circuit breaker in April last year.

This is why Singapore may want to hasten the move to space out the two vaccine doses to enable a larger proportion of the population to get vaccinated.

At least half the people here need to be vaccinated for herd immunity to kick in. The higher the number of people who are vaccinated, the greater the protection for the rest, some of whom are ineligible for vaccinations because of their age or medical conditions.

Said Dr Asok Kurup, chair of the Chapter of Infectious Disease Physicians: "I think it is imperative to get as many people vaccinated, particularly those who are at higher risk but have not received it yet.

"Vaccinated individuals are either asymptomatic or mildly sick, whereas the reverse is true for the unvaccinated."

Added Prof Cook: "I know some of us may have been putting off getting vaccinated because it wasn't so urgent earlier - but it's urgent now.

"If you've been offered vaccination, please don't delay in stepping forward for it."