What you must know about boosters, lapsing of fully vaccinated status

With Singapore bracing itself for a large Omicron infection wave, a person's fully vaccinated status will lapse nine months after the last dose of his or her primary vaccination series in a new policy shift announced on Wednesday.

The Straits Times answers some questions.

A: If you are 18 and older and have been vaccinated with mRNA jabs, you will need to get your booster dose nine months after your second dose to retain your fully vaccinated status. The rules will kick in on Feb 14.

If your second dose was taken before May 20 last year, you will have to get your booster dose before Feb 14 to retain your fully vaccinated status.

A: You are considered fully vaccinated only after three doses. You will still be required to take a booster dose nine months after your third dose to continue being considered as fully vaccinated.

A: Currently, no additional booster doses will be required. However, if you are unvaccinated or partially vaccinated, you will need a dose of mRNA vaccine or two doses of Sinovac or Sinopharm to retain your fully vaccinated status from Feb 14.

For example, if you had taken two doses of Sinovac, and then contracted Covid-19, you would need one more Sinovac or Sinopharm dose post-recovery to qualify as fully vaccinated. You should get your additional dose at least 28 days from the date of your Covid-19 diagnosis.

Those with mixed vaccine combinations should refer to: file.go.gov.sg/sinovac-mixed-vac-combi.pdf

Those who tested positive with an antigen rapid test but did not take a confirmatory polymerase chain reaction test will be required to take a booster dose.

According to the Ministry of Heath's website, there are no additional safety concerns with taking the booster jab if you had unknowingly recovered from a past infection.

There is no known evidence that vaccinating those who have recovered could cause over-stimulation, autoimmune disorders or other safety concerns, it added.

A: Associate Professor Hsu Li Yang, vice-dean of global health and programme leader of infectious diseases at the NUS Saw Swee Hock School of Public Health, said a number of studies in several countries, including Singapore, show that one's neutralising antibodies start falling after two to three months in most vaccinated people. This means that protection against infection would taper off after a few months.

"Protection against severe disease, hospitalisation and death has thankfully lasted longer, no doubt because of T-cells (which kill virus-infected cells) and the adaptive immune system," he added.

The adaptive immune response refers to the second layer of immune defence in the body, which takes days or weeks to develop.

"We currently do not know how long protection against severe Covid-19 disease and death will endure, although that appears to gradually wane over time as well, especially for the elderly and immunosuppressed," noted Prof Hsu.

Agreeing, Professor Ooi Eng Eong, from the Duke-NUS Medical School's emerging infectious diseases programme, said the rate at which immunity wanes is different with different vaccines and in different groups of people, and noted that the benefit of booster vaccination is "less clear at this stage" for the general population and among young adults.

Professor Dale Fisher, a senior consultant from the National University Hospital's infectious diseases division, noted that looking at T-cells and antibodies are "laboratory approaches" to understanding immunity.

"How much does that translate to reality? For this, we need clinical data such as infections, whether mild or severe, for different age groups and whether the patient has an infection after being vaccinated, and how long it has been since they were vaccinated," he added.

Early studies from Britain also suggest that a booster jab will provide 80 to 85 per cent protection against Omicron, compared with 97 per cent against Delta. Though current vaccines are not a perfect match against Omicron, getting vaccinated does make it harder for Omicron to infect the body.

A: Prof Fisher said that it is not completely clear, though there is general consensus that a booster at five to six months is recommended for those at risk of developing severe disease.

"In younger age groups, it is more contentious as their risk of severe disease is less, and this is why a pragmatic longer period is being chosen in Singapore, I believe," he added.

The Expert Committee on Covid-19 Vaccination recommends that people receive their booster dose at about five months after the last dose of their primary vaccination series, or as soon as possible thereafter to maintain good protection against Covid-19.