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US regulators have authorised emergency use of two new boosters that target Omicron variants. Pfizer’s new booster dose, pictured, is authorised for people aged 12 and older and Moderna’s new shot is for adults 18 and older. Photo: Pfizer via AP

ExplainerThe latest coronavirus boosters target Omicron. Are they safe and effective?

  • FDA has authorised emergency use of Pfizer-BioNTech and Moderna boosters that target BA.4 and BA.5 variants of Omicron
  • Shots based on original strain still offer protection but the rapid virus evolution means newer variants can dodge protective antibodies they induce
The US Food and Drug Administration has authorised emergency use of two new Covid-19 vaccines by Pfizer-BioNTech and Moderna that target original strains and the BA.4 and BA.5 variants of Omicron, the strain that currently causes more infections than any others.

This is the first major revamp of Covid-19 vaccines since they were rolled out in December 2020. The boosters, which only went through animal trials and were not tested on humans, have been backed by the US Centres for Disease Control and Prevention, and are expected to be made available to the public in early September.

Here is what you need to know about the updated bivalent booster doses.


Tens of millions under lockdown in China following outbreak of Covid BA.5 subvariant

Tens of millions under lockdown in China following outbreak of Covid BA.5 subvariant

Why do we need bivalent vaccines?

The vaccine shots based on the original strain still offer protection against severe illness and death, but the virus has evolved so much and so fast new variants are able to dodge the protective antibodies induced by the vaccines. As a result, the effectiveness of different vaccines in preventing infection and mild illness has declined substantially. In June, an advisory committee of the US Food and Drug Administration (FDA) voted to include an Omicron component in the new Covid-19 booster vaccines.

They are called bivalent vaccines because they contain two messenger RNA (mRNA) components of Sars-CoV-2 virus – one that was found in the early stages of the pandemic in Wuhan in central China and the basis of vaccines currently in use, and the other BA.4 and BA.5 strains. The latter two have identical spikes.

Public health officials believe the redesigned boosters will provide more lasting protection against the virus and reduce hospital admissions this autumn and winter.

BA.5 may trigger new infection wave in winter: health official

Who should be eligible for them?

Pfizer’s new booster dose is authorised for people aged 12 and older and Moderna’s new shot is for adults 18 and older in the US. The eligible groups can receive the boosters two months after completing their primary series of inoculations or their most recent booster with the older shots.


With approval of these vaccines, the US will no longer use the original vaccines as booster doses, according to a statement by the FDA.

Why are they approved so quickly?

The vaccine developers did not have time to follow the protocols used for the original vaccine and still keep up with the dominant strain.

The FDA reviewed and approved the vaccines based on the available evidence, including the safety and effectiveness data for the original vaccine, safety and immunogenicity data from a clinical trial of other bivalent Covid-19 vaccines that contained mRNA from the Omicron variant BA.1 and the original strain and animal trial data from these two approved vaccines.

“We have worked closely with the vaccine manufacturers to ensure the development of these updated boosters was done safely and efficiently. The FDA has extensive experience with strain changes for annual influenza vaccines. We are confident in the evidence supporting these authorisations,” Peter Marks, director of the FDA’s Centre for Biologics Evaluation and Research, said in a statement.


CDC director Rochelle Walensky told American radio show Conversations on Health Care last week, “If we wait for those data to emerge in human data, not just mice data, we will be using what I would consider to be a potentially outdated vaccine”.

Human studies are planned and will continue even after the boosters are authorised and distributed.

Are the bivalent vaccines safe?

“Given the billions of doses of the original mRNA vaccines that have been given we have a better safety database on these vaccines than in most others. In the case of vaccines of a similar type – mRNA or inactivated virus – where the primary change is the strain, safety is less of an issue,” said Jerome Kim, director general of the International Vaccine Institute.


Individuals who receive a bivalent Covid-19 vaccine may experience the same side effects commonly reported by patients receiving the authorised or originally approved mRNA Covid-19 vaccines, according to the FDA.


Covid-19 still a global health emergency, says WHO as new Omicron subvariants emerge

Covid-19 still a global health emergency, says WHO as new Omicron subvariants emerge

Do they offer better protection?

The FDA said that based on the supporting data, the updated Covid-19 vaccines were expected to provide better protection against the currently circulating Omicron variant.


In June, Pfizer also presented data to the FDA’s independent vaccine advisory committee showing that the Covid-19 shot updated with the BA.5 strain increased protection antibodies in mice by about 2.6-fold compared with the original vaccine.

Kim from the IVI said the new vaccine could likely prevent infection to a small degree and greatly increase protection against severe disease and death in people who were vaccinated more than a year ago, those who had not received a booster or who had not had a recent Covid-19 infection.

“An infection from the summer of 2021 or 2020 are both distant and much less cross protective than the new bivalent vaccine,” Kim said.


However, a study by the University of Washington published in the journal Cell last month found the level of neutralising antibodies induced in mice – against two earlier versions of the coronavirus that are no longer prevalent – could differ from that observed in humans or non-human primates.

Are such vaccines offered elsewhere?

Britain became the first country in the world to approve Moderna’s Omicron-adapted bivalent shot containing the BA.1 strain. The European Medicines Agency (EMA) on Thursday recommended use of the bivalent vaccines by Moderna and Pfizer targeting the original strain and the BA.1 strain for people aged 12 years old or above as a booster dose.

Pfizer said last month it had submitted to the EMA for the same booster dose as that approved in the US for people aged 12 years and older. Mainland China – where less than 60 per cent of the population and only 66 per cent of those aged over 60 had received a booster shot by early August – is yet to approve any foreign vaccine.

Zheng Zhongwei, an official with the National Health Commission in charge of vaccine development, said on Friday that Sinopharm, which had two inactivated Covid-19 vaccines in use, had completed phase 3 human trials of an updated inactivated vaccine targeting the Omicron variant and was filing rolling applications for approval.