The science behind vaccines and how long to wait before being immune

‘Your immune response takes about two to three weeks to produce the antibodies’

Our bodies can take weeks to build protection against Covid-19 after a vaccination jab, Claire O’Connell finds out why it is worth the wait.

“I’m vaccinated!”

The joy, excitement and relief are palpable in the social media posts. It's not surprising that people are celebrating getting "the text" and then the jab, joining the swelling numbers of adults in Ireland who have received their first or second dose of a vaccine against SARS-CoV-2, the virus that causes Covid-19.

And with that relief comes the temptation to consider vaccination as a job done, and to relax the protective behaviours such as good ventilation, wearing masks and avoiding crowds. But not so fast. Getting a vaccine is just one step on a path to building protection against Covid-19, and the process needs time to work.


"After you get the first dose of a Covid-19 vaccine, and if your immune system has never seen this virus before, you are still not protected against the virus for a number of weeks," says immunologist Prof Paul Moynagh from Maynooth University.

“Your immune response takes about two to three weeks to produce the antibodies and T-cells that you need in order to be able to stop the virus from infecting you.”

That weeks-long process involves a complex interplay of cells in your immune system, resulting in tiers of protection from antibodies that can block SARS-CoV-2 from entering your cells, cytotoxic T-cells that find and kill cells in your body that have been infected with the virus and a long-term “memory” of how to tackle SARS-CoV-2.

Antibodies and killer cells

After you get your jab, antibodies appear in two waves over the next few weeks, explains Moynagh. “First you get a type called IgM, they appear from about 10 days on, then they disappear and are replaced in about 14-21 days by IgG antibodies. These are the ones that can stay in your body for months, they can recognise the virus if you are exposed to it, and they bind to the virus, so that it can’t physically get into our cells.”

The newly produced antibodies are even better than the first antibodies by changing so that they stick even more tightly to the virus

Perhaps less celebrated but just as important are the T-cells. Some help the body to make antibodies while others, known as cytotoxic T-cells, develop the capacity to recognise when our own cells have succumbed to the virus and get rid of them.

“If a virus gets into our cells, and the cytotoxic T-cells have been trained by previous exposure to that virus or a vaccine, then the T-cell will kill off that cell, so that the virus can’t replicate inside it. This stops the virus from building up the infection and making us sick,” Moynagh adds.

“Training T-cells is a really important part of the immune response - an antibody might recognise a few parts of a virus, and if those parts of the virus change then the antibody is less effective. But T-cells can still find virus-infected cells even if the virus has changed, so they are likely a good way of protecting against variants, stopping them from making us sick.”

In most cases, the Covid-19 vaccination schedule calls for a second dose. Why? “The first dose will offer you some protection after the immune system has responded to it, but having a second dose after an interval rapidly boosts both the numbers and the quality of your antibodies and your T-cells against the virus, within a week or so,” he explains.

“The newly produced antibodies are even better than the first antibodies by changing so that they stick even more tightly to the virus and more effectively prevent it from infecting our cells. This antibody maturation is the main reason why two doses of the vaccines protect us even better than just one. And in an era where we are seeing variants emerge, we want the best immunity possible.”

Protective memory

After that, our levels of antibodies decline over time. Moynagh believes that vaccine boosters will likely be needed down the line, but he is encouraged by findings that after infection with SARS-CoV-2 or after vaccination against the virus, specialised T-cells and B-cells in the immune system develop a long-term “memory” of SARS-CoV-2.

Prof Kingston Mills is similarly reassured by the persistence of memory T- and B-cells, and how they can jump into action if needed. "They start cranking up really quickly if you get a challenge by the virus, and the B-cells start making antibody rapidly," he says. "So even if the antibodies have declined below a critical, non-protective threshold, you could be protected through the re-activation of these memory B-cells."

Numbers game

Building immunity against Covid-19 is "a numbers game", according to Mills, who is professor of experimental immunology at Trinity College Dublin, and this is why we need to have as much protection as possible in the face of both the original SARS-CoV-2 and the the variants where the virus has changed and may present a slightly different "look" to the immune system.

“Variants have different structures and sequences, so if you have been vaccinated against the original strain, then the antibodies you have made could be less capable of neutralising the variant virus, particularly after the primary or first dose,” he says. “This is where it is a numbers game, and after two doses of the vaccine you will generate more neutralising antibodies in about seven days, and they will be broader in specificity, so they may offer wider protection even against variants.”

While the vaccination programme here has been doing a brilliant job, we still have vulnerable people waiting for their vaccines

There is no cutting corners on waiting for the body to respond to vaccines, says Christine Loscher, professor of immunology at Dublin City University. "After you get a vaccine, you are turning on different parts of your immune system and it takes weeks. Then the second dose supersizes your immune response, and gives you better and longer-lasting protection, which is really important because now we are dealing with the emergence of variants as well as the original virus."

Protect against delta

The variant that is particularly focusing the mind in Ireland at the moment is the delta variant. It was initially reported in India and now accounts for a large proportion of cases in the UK, where evidence shows that having two doses of the Pfizer or AstraZeneca vaccine offers protection against hospitalisations from the variant.

“The delta variant has upset the apple cart,” says Loscher. “A partial dose of a Covid-19 vaccine may offer only 30 per cent protection against the delta variant, so it is really important to stay safe and make sure you get all the doses and give your body time to build its immune response to protect you. Also the delta variant is highly transmissible, so even if you have had a vaccine and you have enough protection to stop you getting sick, you might be still able to carry the virus and transmit it to others. We don’t have enough data on delta to be able to say the vaccine, even with a second dose, reduces the transmissibility significantly.”

Loscher notes vaccines work best when more people have them, reducing the possibility for the virus to find unprotected hosts. “While the vaccination programme here has been doing a brilliant job, we still have vulnerable people waiting for their vaccines, and this combined with the potential for the delta variant to throw a spanner in the works means we need to be really careful,” she says.

“We are in a relatively good position to keep a close eye on the delta variant here, we know about it, and as individuals I think we can all play a role in keeping safe, ensuring we are outdoors for dining out or meeting up with people, getting the full doses of vaccinations and giving them time to work and continuing to be careful, particularly when mixing with people who have not been vaccinated.”

Claire O'Connell

Claire O'Connell

Claire O'Connell is a contributor to The Irish Times who writes about health, science and innovation