SINGAPORE - 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 primary vaccination series, in a new policy shift announced on Wednesday (Jan 5).
The Straits Times answers some questions.
Q: How long does my fully vaccinated status last if I had been vaccinated with an mRNA vaccine? When do the rules kick in?
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.
Q: How long does my fully vaccinated status last if I had taken the Sinovac or Sinopharm vaccines?
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 fully vaccinated.
Q: Do I still need to take a booster dose if I have recovered from Covid-19?
A: Currently, no additional booster doses will be required. However, if you are unvaccinated or partially vaccinated, you will need a dose of an 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 this link.
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 Health’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 overstimulation, autoimmune disorders or other safety concerns, it added.
Q: What is the rationale behind the nine-month expiry date of one's fully vaccinated status? Will my protection against Covid-19 wane by then? Will we need multiple booster shots in the future?
A: Associate Professor Hsu Li Yang, vice-dean of global health and programme leader of infectious diseases at the National University of Singapore's Saw Swee Hock School of Public Health, said a number of studies in several countries, including Singapore, show that in most vaccinated people, their neutralising antibodies start falling after two to three months. 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 in immunity wanes is different with different vaccines and in different groups of people, and noted that the benefit for booster vaccination is "less clear at this stage" for the general population and among young adults.
"If the goal is to prevent any infection, then it is likely that we will require multiple booster shots to sustain the high levels of antibodies needed to block infection. If the goal is to prevent all symptomatic infection, including those that are mild, then annual or even six-monthly vaccination may be needed as again, the baseline level of immune activity against Sars-CoV-2 will need to be high," said Prof Ooi.
"If, however, the goal is to reduce the risk of severe Covid-19 that needs hospitalised management, then there is little indication that vaccine-induced immunity has waned to those levels in healthy young adults."
Professor Dale Fisher, a senior consultant at 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.
The vulnerable, such as seniors, are at higher risk, which is why a booster is recommended at five to six months after the primary vaccination series.
"We know at this time we see more mild infections, and there is no appetite to wait longer to see if severe infections emerge," he added.
Early studies from Britain also suggested that a booster jab will provide 80 per cent 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.
More antibodies are developed, thanks to the booster, giving the body stronger defences against the virus.
Q: Are there any benefits in taking my booster dose nine months after my second dose, rather than at the fifth- or sixth-month mark?
A: Prof Fisher said 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.
Q: What are other countries doing in terms of booster jabs?
A: Israel was the first country to roll out booster jabs for those who took the Pfizer vaccine, and mandates that those 12 and older must take a booster dose to be considered fully vaccinated. It is now considering a second booster dose for those 60 and older.
The United States Centres for Disease Control and Prevention (CDC) said three doses of Pfizer-BioNTech's or Moderna's vaccines should be considered "up-to-date" inoculations, and that Johnson & Johnson recipients should receive a second dose, preferably of Moderna or Pfizer-BioNTech, to also be considered up to date. The CDC also expanded its recommendation for booster shots to include all Americans 12 and older.
Though the CDC did not change the technical definition of "fully vaccinated' - two doses of an mRNA vaccine or one dose of the J&J vaccine - the agency did recommend that people "stay 'up to date' by receiving any additional doses they are eligible for to ensure optimum protection".
In Britain, boosters are available to all above 18, three months after their second dose.