The effectiveness of the Covid vaccine remains high in the first two months after receiving a booster, and then begins to wane, a report published by the US Centers for Disease Control and Prevention (CDC) has found, providing insight into how immunity against the virus fares after a third shot.

The CDC report says that during the Omicron-predominant period, vaccine effectiveness against emergency department visits and hospitalisation was 87% and 91%, respectively, in the two months after a third dose.

By the fourth month after a booster dose, this decreased to 66% and 78% respectively, which is still strong protection. By comparison, Australian data suggests influenza vaccination is associated with a 31% reduction in influenza-related deaths, though this changes depending on influenza strain and vaccine.

So, will we need more Covid-19 shots?

It is still early days into Omicron, and we will need several months more data to determine if and how often further shots may be needed. Fourth doses are already recommended in Australia for the immunocompromised, who produce a lower immune response than the equivalent number of doses in healthy people.

Most countries, including the US, Chile, and the UK, are taking a similar approach to Australia, focussing fourth doses on the immunocompromised. Israel, which began administering third doses in July, is the first country to administer a fourth dose more widely. There, anyone who is a health worker, immunocompromised, or over 60 has been eligible for a fourth dose since January, and the government is considering expanding eligibility to all adults even in the absence of strong data to support this.

Preliminary data from Israel suggests among adults aged 60 and older, the fourth dose raises protection against severe illness by up to three times compared with those vaccinated with a third dose.

But Dr Michelle Willicombe, from Imperial College London’s department of immunology, told the British Medical Journal that even if a fourth dose proves to further boost antibodies in healthy people, it does not mean it will be necessary, as other aspects of the immune system, aside from those induced by vaccination, also offer protection.

“If you give someone a boost, and they’ve got a normal immune system, they’re going to get a boost in response,” she said. “The question is, is it needed? I don’t know the answer to that … the clinical efficacy data, not just infection rates but hospital admissions, deaths, etc, would guide that.”

There is also a lack of safety data for a fourth dose and, while it is likely to be safe given what we know about third doses, Australian regulators require strong data before approving additional doses.

If the vaccine is so great, why do I need a booster?

It is important to recognise that it is very rare for any vaccine to provide sterilising immunity; that is, enabling the immune system to completely prevent a virus from replicating in and infecting the body.

Measles, pertussis (whooping cough), polio and likely Covid-19 vaccines are all examples of vaccines that do not provide sterilising immunity. Yet measles and polio have still been eliminated in many countries, and their disease burden worldwide has been significantly reduced. This is because, just like the Covid-19 vaccines, the vaccines for these illnesses are extremely good at preventing severe disease and death.

However, three doses of the Covid-19 vaccines are needed to get this strong protection against severe disease and death, particularly from the Omicron strain.

“Vaccine effectiveness against symptomatic disease with the Omicron variant is substantially lower than against the Delta variant, with rapid waning,” data from the UK Health Security Agency says. “However, protection against hospitalisation remains high, particularly after three doses.”

If it does turn out that more than three doses of Covid-19 vaccines are eventually recommended, it will not be unusual. The whooping cough vaccine, for example, is a five-dose course. The flu vaccination is recommended every year, although it is tweaked to target influenza strains predicted to circulate that season.

Ass Prof Sanjaya Senanayake, an infectious diseases specialist at the Australian National University, said if a fourth dose is required more broadly, the preference would be for a variant-specific one.

“Right now, both Pfizer and Moderna have updated their formulations to develop an Omicron-specific vaccine,” he said. “If the vaccines do well in trials, they will become available to the general public, but this may not be until the second half of the year. Hopefully the combination of natural immunity from these Omicron waves and lingering booster immunity, in combination with commonsense Covid-safe measures, will protect us till then.”

Until then, it is sensible to continue to target a fourth dose at vulnerable populations, such as the immunocompromised, he said.

Just over 51% of Australians over the age of 18 have received their third dose.

Better approaches than boosters?

While such a high proportion of the world has woeful access to vaccination, there is a higher risk of new variants emerging, including those that can evade available vaccines.

Though vaccination is vital, countries like Australia would be better protected against new Covid variants in the long term if they also donated more vaccine doses to less wealthy countries, research shows.

Post Source Guardian