Why is there a gap between 2 doses of coronavirus vaccine?

Krishna: These things are decided by research based evidence. Different gap periods are suggested for different vaccines based on the efficacy results. Some vaccines, like the Johnson and Johnson vaccine, are efficient even with one dose. Some like covishield, covaxin, Sputnik V, Pfizer-BioNTech and Moderna need two doses. The timeframe between the doses depends on which vaccine you received.

Why do some vaccines require two doses? How long do you need to wait between doses? Do you have any immunity after the first dose?

Vaccine efficacy refers to how well it performs in a carefully controlled trial, whereas effectiveness describes its performance in the real world.

The Pfizer-BioNTech and Moderna vaccines both use mRNA technology. The cells in your body naturally use mRNA all the time to create many different types of proteins that you need to stay healthy. The mRNA in the two-dose COVID-19 vaccines provide the cells of your body with instructions on how to make the spike protein that’s specific to the new coronavirus. This spike protein is found on the surface of the virus. It uses this spike protein to attach to and enter a host cell in your body.

The first time you give someone a dose of vaccine against a particular infection, it’s called a prime. You’re getting your immune response ready to roll.

Each time you give another dose against that same infection, it’s called a boost. You’re building on immunity you already have from the first dose.

Importantly, giving smaller doses in multiple shots is often better than a large dose of vaccine in a single shot. This is because our immune system builds on our immunity like bricks in a wall; each level needs to be laid before the next layer is built.

Booster shots take advantage of a phenomenon called “immunological memory”. Our immune cells essentially remember vaccines we’ve previously received, and respond much more quickly and vigorously to subsequent shots, building our immunity to levels at which we can be confident we’ll be protected.

When you get your COVID-19 vaccine, your immune system processes information on the spike protein and generates an immune response to it. This includes the production of antibodies. Vaccines hinge on a very important aspect of your immune system, which is the fact that it has memory. After your vaccination, your immune system continues to store information on the spike protein. If you’re exposed to the new coronavirus, your immune system will then use this stored information to respond and protect you from the virus. This can help prevent you from becoming sick with COVID-19. The entire immunity process typically takes around 2 weeks after the second dose of vaccine. That’s why you can still contract the new coronavirus and become ill if you’re exposed to it shortly after getting your vaccine.

Why do you need this waiting period?

The first dose helps to prime your immune system. It introduces it to the spike protein and allows it to generate a small immune response prior to the second dose. Time is needed to allow this process to develop properly. It’s also important not to get your second dose too early or too late, as this may impact the effectiveness of the vaccine. You have to stick to the prescribed period gap.

You do get some immunity after you receive the first dose. But it will not be full. It will usually be less. This depends on the vaccine, your immune response, and the time period.

The effectiveness increases for most Covid vaccines only after the second dose. Clinical researchers observed a strong antibody response following the second dose. But still we don’t yet know how long this immunity lasts.

Some vaccines require two doses because the immune response to the first dose is rather weak. The second dose helps to better reinforce this immune response.

If you don’t take the second dose when it is suggested, it’s possible that overall immunity or vaccine effectiveness could be impacted. Additionally, lower levels of immunity could drive the emergence of more virus variants that can escape our current vaccines.

While this is the main reason for the gap, Increasing gap can help more people receive jabs, according to some governments and policy makers.
According to experts, increasing the interval between the vaccine jabs will allow the government to give doses to more people as well as increase exports of vaccines.

When do you need a booster dose?

There are three different situations in which you might need a boost (1).

First, several doses of a vaccine can be given relatively quickly, one after another, to rapidly build someone’s immunity against a given infection. A good example is the whooping cough vaccine. It’s initially given at around two, four and six months of age to rapidly build immunity in infants, who are most at risk from whooping cough.

This is also the approach most COVID vaccines use. The first shot gets your immune system going but immunity is unreliable. The second shot leads to more consistent protection.

Second, we can give a booster shot if immunity drops over time, or “wanes”, to restore someone’s immunity to optimal levels. For example, we know immunity to tetanus can drop over time, so we recommend tetanus boosters every ten years.

Immunity appears to be strong three months after the Moderna vaccine and six months after the AstraZeneca vaccine, but we don’t yet have a full picture of how long immunity to COVID-19 lasts after vaccination. Scientists will continue to monitor this to determine if and when we’ll need these type of boosters for COVID.

Third, if the virus “mutates” or changes substantially over time, this can make it challenging for our immune cells to recognise the virus, effectively lowering our immunity again. A good example here is the influenza vaccine. The ‘flu virus can change a lot from year to year so, to make sure immunity remains high, we give annual boosters tailored to new strains.

With boosting, you can end up with a higher level of immunity if you wait longer between doses. This is because our immune cells need a rest before they can respond to additional doses. We’ve seen this with the AstraZeneca vaccine where a longer delay between doses, up to 12 weeks, leads to much better protection.

It’s also possible we could generate greater immunity if we use different vaccines, one after the other, rather than repeating the same vaccine. This is called heterologous prime boosting.

We’re not sure why a mix-and-match approach can be more potent. But it’s possible combining two different vaccines — which give the same antigen target but stimulate the immune system in different ways — could better focus our immune cells’ attention on the right target.