Australia’s Therapeutic Goods Administration (TGA) has recently approved a new covid booster. The shot was developed by Pfizer and targets the JN.1 sub-variant of Omicron.

This is now the fifth iteration of the covid vaccines, which have been updated regularly to keep up with the rapidly evolving virus, SARS-CoV-2.

But nearly five years into the pandemic, you may be wondering, why do we need yet another type of covid booster? And do we still need to be getting boosters at all? Here’s what to consider.

Targeting the spike protein

Pfizer’s JN.1 booster (and Moderna’s, though the TGA has not approved this one at this stage) is based on mRNA technology. This technology instructs our cells to produce a specific protein – in this case SARS-CoV-2’s spike, a protein on the surface of the virus that allows it to attach to our cells.

This helps the immune system produce antibodies that recognise the spike protein and interfere with the virus getting into our cells.

In response to our strengthened immune responses from vaccinations and previous infections (called immune pressure), SARS-CoV-2 has continued to evolve over the course of the pandemic, modifying the shape of its spike protein so our antibodies become less effective.

Most recently we’ve faced a soup of Omicron sub-variants, including JN.1. Since JN.1 was first detected in August 2023, this Omicron sub-variant has spawned a variety of further sub-variants, such as KP.2 (known as FLiRT), KP.3 (known as FLuQE) and XEC.

The spike protein is made up of 1,273 amino acids, a bit like molecular building blocks. Mutations to the spike protein change individual amino acids.

Certain amino acids are important for allowing neutralising antibodies to bind to the spike protein. This means changes can give the virus an edge over earlier variants, helping it evade our immune response.

Scientists keep updating the covid vaccines in an effort to keep up with these changes. The better matched the vaccine “spike” is to the spike protein on the surface of the virus trying to infect you, the better protection you’re likely to get.

So who should get vaccinated, and when?

Updating vaccines to deal with mutating viruses is not a new concept. It has been happening for the flu vaccine since around 1950.

We’ve become accustomed to getting the annual flu vaccine in the lead-up to the winter cold and flu season. But, unlike influenza, covid has not settled into this annual seasonal cycle. The frequency of covid waves of infection has been fluctuating, with new waves emerging periodically.

Covid is also more transmissible than the flu, which presents another challenge. While numbers vary, a conservative estimate of the reproduction number (R0 – how many people will one person will go on to infect) for JN.1 is 5. Compare this to seasonal flu with an R0 of about 1.3. In other words, covid could be four times more transmissible than flu.

Add to this immunity from a covid vaccination (or a previous infection) begins to wane in the months afterwards.

So an annual covid booster is not considered enough for some more vulnerable people.

For adults aged 65 to 74, a booster is recommended every 12 months, but they’re eligible every six months. For adults over 75, a shot is recommended every six months.

Adults aged 18 to 64 are eligible every 12 months, unless they have a severe immune deficiency. Many conditions can cause immunodeficiency, including genetic disorders, infections, cancer, autoimmune diseases, diabetes and lung disease, as well as having received an organ transplant. For this group, it’s recommended they receive a shot every 12 months, but they’re eligible every six.

Making sense of the advice

A vaccine that targets JN.1 should provide good protection against the Omicron sub-variants likely to be circulating in the coming months.

A few things need to happen before the JN.1 shots become available, such as the Australian Technical Advisory Group on Immunisation providing guidance to the government. But we can reasonably expect they might be rolled out within the next month or so.

If they hit doctors’ offices and pharmacies before Christmas and you’re due for a booster, the holiday period might be added impetus to go and get one, especially if you’re planning to attend lots of family and social gatherings over summer.

In the meantime, the XBB.1.5 vaccines remain available. Although they’re targeted at an earlier Omicron sub-variant, they should still offer some protection.

While young, healthy people might like to wait for the updated boosters, for those who are vulnerable and due for a vaccination, whether or not to hold out may be something to weigh up with your doctor.

The advice on covid boosters in Australia, with stronger wording (“recommended” versus “eligible”) used for more vulnerable groups, reflects what we know about covid. People who are older and medically vulnerable are more likely to become very unwell with the virus.

For young, healthy people who may be wondering, “do I need a covid booster at all?”, having one annually is sensible. Although you’re less likely to get very sick from covid, it’s possible. And, importantly, vaccines also reduce the risk of developing long covid.

While covid vaccines do a very good job of protecting against severe disease, they don’t necessarily stop you becoming infected. Evidence on whether they reduce transmission has been mixed, and changed over time.

We’ve come to appreciate that vaccination is not going to free us of covid. But it’s still our best defence against severe illness.

Nathan Bartlett, Professor, School of Biomedical Sciences and Pharmacy, University of Newcastle. Republished  from The Conversation.