Almost 4 years into the pandemic, Australia, like many different international locations, remains to be seeing massive numbers of COVID instances. Some 860,221 infections have been recorded across the nation in 2023, whereas 30,283 instances have already been reported in 2024.
That is more likely to be a big underestimate, with fewer folks testing and reporting than earlier within the pandemic. However the indicators recommend elements of Australia are experiencing but one other COVID surge.
Whereas some fortunate folks declare to have by no means had COVID, many are dealing with our second, third and even fourth an infection, usually regardless of having been vaccinated. You could be questioning, how lengthy does immunity final after a earlier an infection or vaccination?
Let’s check out what the proof reveals.
B cells and T cells
To reply this query, we have to perceive a bit about how immunity to SARS-CoV-2 (the virus that causes COVID) works.
After being contaminated or vaccinated, the immune system develops particular antibodies that may neutralise SARS-CoV-2. B cells keep in mind the virus for a time period. As well as, the immune system produces reminiscence T cells that may kill the virus, and stay within the blood for some months after the clearance of the an infection or a vaccination.
A 2021 examine discovered 98% of individuals had antibodies in opposition to SARS-CoV-2’s spike protein (a protein on the floor of the virus that permits it to connect to our cells) one month after symptom onset. Six to eight months afterwards, 90% of individuals nonetheless had these neutralising antibodies of their blood.
This implies the immune system ought to have recognised and neutralised the identical SARS-CoV-2 variant if challenged inside six to eight months (if an an infection occurred, it ought to have resulted in delicate to no signs).
Learn extra: What occurs in our physique after we encounter and struggle off a virus just like the flu, SARS-CoV-2 or RSV?
However what about when the virus mutates?
As we all know, SARS-CoV-2 has mutated over time, resulting in the emergence of recent variants resembling alpha, beta, delta and omicron. Every of those variants carries mutations which can be new to the immune system, even when the individual has been beforehand contaminated with an earlier variant.
A brand new variant probably will not be completely recognised – and even recognised in any respect – by the already activated reminiscence T or B cells from a earlier SARS-CoV-2 an infection. This might clarify why folks may be so readily reinfected with COVID.
A latest overview of research revealed as much as the top of September 2022 regarded on the safety conferred by earlier SARS-CoV-2 infections.
The authors discovered a earlier an infection offered protecting immunity in opposition to reinfection with the ancestral, alpha, beta and delta variants of 85.2% at 4 weeks. Safety in opposition to reinfection with these variants remained excessive (78.6%) at 40 weeks, or simply over 9 months, after the earlier an infection. This safety decreased to 55.5% at 80 weeks (18 months), however the authors famous there was a scarcity of knowledge at the moment level.
Notably, an earlier an infection offered solely 36.1% safety in opposition to a reinfection with omicron BA.1 at 40 weeks. Omicron has been described as an immune escape variant.
A previous an infection confirmed a excessive degree of safety in opposition to extreme illness (above 88%) as much as 40 weeks whatever the variant an individual was reinfected with.
Learn extra: There are nonetheless good causes to keep away from catching COVID once more – for one, your danger of lengthy COVID goes up every time
What about immunity after vaccination?
Thus far nearly 70 million COVID vaccines have been administered to greater than 22 million folks in Australia. Scientists estimated COVID vaccines prevented round 14.4 million deaths in 185 international locations within the first yr after they turned obtainable.
However we all know COVID vaccine effectiveness wanes over time. A 2023 overview discovered the unique vaccines have been 79.6% and 49.7% efficient at defending in opposition to symptomatic delta an infection at one and 9 months after vaccination respectively. They have been 60.4% and 13.3% efficient in opposition to symptomatic omicron on the similar time factors.
That is the place booster doses come into the image. They’re necessary to maintain the immune system able to struggle off the virus, notably for many who are extra susceptible to the consequences of a COVID an infection.
Plus, common booster doses can present immunity in opposition to totally different variants. COVID vaccines are continually being reviewed and up to date to make sure optimum safety in opposition to present circulating strains, with the most recent shot obtainable designed to focus on the omicron variant XBB 1.5. That is much like how we strategy seasonal flu vaccines.
A latest examine confirmed a COVID vaccination supplies longer safety in opposition to reinfection than pure safety alone. The median time from an infection to reinfection in non-vaccinated folks was solely six months, in contrast with 14 months in individuals who had acquired one, two or three doses of vaccine after their first an infection. That is known as hybrid immunity, and different analysis has equally discovered it supplies higher safety than pure an infection alone.
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It additionally appears timing is necessary, as receiving a vaccine too quickly after an an infection (lower than six months) seems to be much less efficient than getting vaccinated later.
What now?
Everybody’s immune system is barely distinctive, and SARS-CoV-2 continues to mutate, so understanding precisely how lengthy COVID immunity lasts is sophisticated.
Proof suggests immunity following an infection ought to typically final six months in wholesome adults, and may be extended with vaccination. However there are exceptions, and all of this assumes the virus has not mutated a lot that it “escapes” our immune response.
Whereas many individuals really feel the COVID pandemic is over, it is necessary we do not overlook the teachings we have now discovered. Practices resembling sporting a masks and staying dwelling when unwell can scale back the unfold of many viruses, not solely COVID.
Vaccination shouldn’t be necessary, however for older adults eligible for a booster underneath the present tips, it is an excellent concept.
Lara Herrero, Analysis Chief in Virology and Infectious Illness, Griffith College
Wesley Freppel, Analysis Fellow, Institute for Glycomics, Griffith College
