newsGP -How long does immunity last after a COVID infection? – RACGP

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Dr Lara Herrero and Wesley Freppel assess the latest evidence, including the impact vaccination and emerging variants can have.

Nearly four years into the pandemic, Australia, like many other countries, is still seeing large numbers ofCOVID cases. Some 860,221 infections were recorded around the country in 2023, while 30,283 cases have already been reported in 2024. This is likely to be a significant underestimate, with fewer people testing and reporting than earlier in the pandemic, but the signs suggest parts of Australia are experiencing yetanother COVID surge. While some lucky people claim to have never had COVID, many are facing our second, third or even fourth infection, often despite having been vaccinated. You might be wondering, how long does immunity last after a previous infection or vaccination? Lets take a look at what the evidence shows. B cells and T cells To answer this question, we need to understand a bit about howimmunityto SARS-CoV-2 works. After being infected or vaccinated, the immune system develops specific antibodies that can neutralise SARS-CoV-2. B cells remember the virus for a period of time. In addition, the immune system produces memory T cells that can kill the virus and remain in the blood for some months after the clearance of the infection or a vaccination. A2021 studyfound 98% of people had antibodies against SARS-CoV-2s spike protein (a protein on the surface of the virus that allows it to attach to our cells) one month after symptom onset. Six to eight months afterwards, 90% of participants still had these neutralising antibodies in their blood. This means the immune system should have recognised and neutralised the same SARS-CoV-2 variant if challenged within 68 months (if an infection occurred, it should have resulted in mild to no symptoms). But what about when the virus mutates? As we know, SARS-CoV-2 has mutated over time, leading to the emergence of new variants such as Alpha, Beta, Delta and Omicron. Each of these variants carries mutations that are new to the immune system, even if the person has been previously infected with an earlier variant. A new variant likely wont beperfectly recognised or evenrecognised at all by the already activated memory T or B cells from a previous SARS-CoV-2 infection. This could explain why people can be so readily reinfected with COVID. A recentreview of studiespublished up to the end of September 2022 looked at the protection conferred by previous SARS-CoV-2 infections. The authors found a previous infection provided protective immunity against reinfection with the ancestral, Alpha, Beta and Delta variants of 85.2% at four weeks. Protection against reinfection with these variants remained high (78.6%) at 40 weeks, or just over nine months, after the previous infection. This protection decreased to 55.5% at 80 weeks (18 months), but the authors noted there was a lack of data at this time point. Notably, an earlier infection provided only 36.1% protection against a reinfection with Omicron BA.1 at 40 weeks. Omicron has been described as animmune escape variant. A prior infection showed a high level of protection against severe disease (above 88%) up to 40 weeks, regardless of the variant a person was reinfected with. What about immunity after vaccination? So far, almost 70 million COVID vaccineshave been administeredto more than22 million peoplein Australia. Scientists estimated COVID vaccines prevented around14.4 million deathsin 185 countries in the first year after they became available. But we know COVID vaccine effectiveness wanes over time. A2023 reviewfound the original vaccines were 79.6% and 49.7% effective at protecting against symptomatic Delta infection at one and nine months after vaccination, respectively. They were 60.4% and 13.3% effective against symptomatic Omicron at the same time points. This is where booster doses come into the picture. Theyre important to keep the immune system ready to fight off the virus, particularly for those who are more vulnerable to the effects of a COVID infection. Plus, regular booster doses can provide immunity against different variants. COVID vaccines are constantly beingreviewed and updatedto ensure optimal protection againstcurrent circulating strains, with the latest shot available designed to targetthe Omicron variant XBB 1.5. This is similar to how we approach seasonal flu vaccines. Arecent studyshowed a COVID vaccination provides longer protection against reinfection than natural protection alone. The median time from infection to reinfection in non-vaccinated people was only six months, compared with 14 months in people who had received one, two or three doses of vaccine after their first infection. This is calledhybrid immunity, and other research has similarly found it provides better protection than natural infection alone. It also seems timing is important, as receiving a vaccine too soon after an infection (less than six months) appears to beless effectivethan getting vaccinated later. What now? Everyones immune system is slightly unique, and SARS-CoV-2 continues to mutate, so knowing exactly how long COVID immunity lasts is complicated. Evidence suggests immunity following infection should generally last six months in healthy adults and can be prolonged with vaccination. But there are exceptions, and all of this assumes the virus has not mutated so much that it escapes our immune response. While many people feel the COVID pandemic is over, its important we dont forget the lessons we have learned. Practices such as wearing a mask and staying home when unwell can reduce the spread of many viruses, not onlyCOVID. Vaccination is not mandatory, but for older adults eligible for a booster under thecurrent guidelines, its a very good idea. Log in below to join the conversation. First published inThe Conversation. Read theoriginal article.

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newsGP -How long does immunity last after a COVID infection? - RACGP