While the evidence is still emerging, preliminary data suggests a Pfizer booster might give the same protection against Omicron as double-dose vaccination did for the original strain.
Why get a booster?
When you get your first dose of COVID vaccine, your body produces an immune response against a part of the virus called the spike protein. If you're exposed to the SARS-CoV-2 virus, your immune system can recognise and fight the virus quickly.
The immune response to a single dose of COVID vaccine is generally short-lived. So a second dose is needed to have a stronger and longer-lasting response.
Over time, the amount of antibodies in your body decreases – this is referred to as waning immunity.
If the immune response wanes below the level needed for protection against COVID – the "protective threshold" – your immune system may not be able to prevent infection when exposed to the virus.
Vaccine doses given some time after the initial course help boost the level of antibodies above the protective threshold.
How much does immunity wane after two doses?
Antibodies decrease over a period of six months or more after the second dose of COVID vaccine.
Vaccine effectiveness against COVID infection decreases, on average, by 18.5 percentage points, six months after completing vaccination.
On a positive note, protection against serious COVID illness, including hospitalisation or death, does not seem to be reduced to the same extent, only by about 8 percentage points.
This is likely because other components of the immune response (T cells and immune memory cells) stay in the body for longer than antibodies and prevent serious illness.
Waning protection is more of a concern among elderly and immunocompromised people because they tend to have weaker immune responses to vaccines compared with young, healthy people.
How effective are booster doses?
Although protection against COVID infection from two doses was slightly lower against Delta than the original strain, a booster dose restores protection to the same level.
In Israel, people who received a booster dose (five or more months after completing vaccination) had infection rates ten times lower than in people who had only received the initial two-dose course.
From a safety perspective, the types and frequency of side effects after the booster dose have been similar to first and second doses.
Which vaccine should I get as my booster dose?
A recent clinical trial showed several COVID vaccines, including all three currently available in Australia (Pfizer, Moderna and AstraZeneca), and the Novavax and Janssen vaccines, produce strong immune responses after a course of either Pfizer or AstraZeneca vaccines.
Based on what we know so far about immune responses to COVID vaccines, any of these vaccines given as a booster should be effective in reducing your risk of infection, regardless of which vaccine you initially received.
The highest immune responses were seen with mRNA vaccines, but it's too early to tell whether these provide better protection against COVID infections when used as a booster, or how quickly immune responses will wane compared with the other vaccines.
When is the best time to get my booster dose?
Booster doses are timed to boost your antibody levels before they get below the protective threshold. The difficulty with COVID is we don't yet know what the protective immune threshold is.
So the timing also involves other factors such as how much disease is in the community and vaccine availability. Some countries, such as the United Kingdom, have recommended getting a booster dose as soon as three months after the initial course.
The UK has much higher daily cases of COVID, and face the potential for increased Omicron cases in winter, when hospitals are often at capacity due to other common respiratory viruses including influenza. In that context, early boosters are like an insurance policy to prevent an overwhelming winter peak.
With this vaccination schedule, most adults in Australia will be eligible for their booster before winter 2022.
Will the booster protect me against Omicron?
Early laboratory studies show two doses of the Pfizer vaccine provide some immunity against Omicron, but not as much as against previous strains. This means we're likely to see more infections in fully vaccinated people.
However a booster dose appears to improve the immune response to a level similar to that observed against previous strains in fully vaccinated people, and is expected to provide good protection against serious illness.
As more data on the effectiveness of boosters emerges, and if Omicron cases increase rapidly, the recommended timing of booster doses may also change.
While we wait for more data to confirm the vaccines provide good protection against hospitalisation and death, we can take some comfort knowing early data indicate this variant may even be less severe than previous ones.
In the future, booster doses may be adapted for emerging variants, much like influenza vaccines are modified each year depending on what new strains are circulating.
The benefits of new vaccine technologies like mRNA is the time required to manufacture new variant vaccines is only about 100 days. So if a vaccine-resistant variant does arise, we might not need to wait too long for an updated vaccine.
Authors: Cyra Patel, Senior Research Officer, National Centre for Immunisation Research and Surveillance; PhD candidate, Australian National University; Jean Li-Kim-Moy, General Paediatrician; Research Fellow, National Centre for Immunisation Research and Surveillance, University of Sydney, and Robert Booy, Infectious Disease Paediatrician and Senior Professorial Fellow, National Centre for Immunisation Research and Surveillance, University of Sydney
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