Many questions remain about both natural and vaccine induced immunity to SARS-CoV-2

There is reason to think that immunity could last for several months or a couple of years, at least, given what we know about other viruses and what we have seen so far in terms of antibodies in patients with covid-19 and in people who have been vaccinated. However, in some studies, for example, the longevity of antibodies targeting the spike of SARS-CoV-2 is shorter than one might expect. So no definite number or months or years cannot be given at this point in time.

Immunity is also determined by other factors besides antibodies, such as T and B cell memory, which some studies estimate could last for years.2 And immunity is induced differently by natural infection versus vaccination, so one can’t just combine studies to arrive at a definitive figure.

How long do antibodies against covid-19 stay in the body?

One study, published in the journal Immunity, of 5882 people who had recovered from covid-19 infection, found that antibodies were still present in their blood five to seven months after illness.3 This was true for mild and severe cases, though people with severe disease ended up with more antibodies overall.

Reports from Vaccine manufacturers themselves: Moderna – high levels of antibodies six months after their second dose. Astra Zennica – induced high antibodies with “minimal waning” for three months after a single dose

Neutralising antibodies are expected to decline in number over time given what we know from responses to other infections. Researchers looked at antibody levels and functions in people who had experienced symptomatic or asymptomatic covid. Both types of participant possessed polyfunctional antibodies, which can neutralise the virus or assist in killing infected cells, among other things. 

A modelling study published in Nature Medicine examined the decay of neutralising antibodies for seven covid-19 vaccines. The authors argued that “even without immune boosting, a significant proportion of individuals may maintain long-term protection from severe infection by an antigenically similar strain, even though they may become susceptible to mild infection.

What about T and B cell responses?

T and B cells have a central role in fighting off infections and, crucially, in establishing long term immunity. Some T and B cells act as memory cells, persisting for years or decades, primed and ready to reignite a broader immune response should their target pathogen arrive in the body again. It’s these cells that make truly long term immunity possible. Some experts say that “we have evidence that T and B cells can confer lifelong protection against certain diseases similar to covid-19”.

There’s a conundrum, for example, in knowing that T cells help B cells to rapidly make high affinity antibodies on re-exposure. How much does it matter that serum antibodies have a short life and wane rapidly, if the cells making them are established and ready to go?

How does natural immunity compare with vaccine induced immunity?

Various studies have shown that an immune response involving memory T and B cells emerges after covid-19 infection.11 But people’s immune systems tend to respond in very different ways to natural infection, Data from the clinical trials of the leading vaccine candidates have found T and B cell reactivity

Does vaccination make a difference to those who have already had covid-19?

There is some evidence that vaccination can sharpen immunity in people who have previously been infected with SARS-CoV-2 and recovered. A letter published in the Lancet in March discussed an experiment in which 51 healthcare workers in London were given a single dose of the Pfizer vaccine. Half of the healthcare workers had previously recovered from covid-19 and it was they who experienced the greatest boost in antibodies—more than 140-fold from peak pre-vaccine levels—against the virus’s spike protein

Is there any difference in vaccine induced immunity between the first and second doses?

It’s difficult to get a sense of the entire immune response after one dose of vaccine versus two, but multiple studies have investigated antibody levels at different stages of dosing. One preprint study from researchers at University College London involving more than 50 000 participants found that 96.4% were antibody positive one month after their first dose of either the Pfizer or AstraZeneca vaccines, and 99.1% were antibody positive between seven and 14 days after their second dose. Median antibody levels changed slightly up to two weeks after the second dose, at which point they rocketed

Another study, also a preprint by researchers in the UK, evaluated the difference in peak antibody levels among 172 people over 80 who received the Pfizer vaccine.16 Those who had no previous record of covid-19 infection had 3.5 times more antibodies at their peak if they received their second dose 12 weeks later rather than three weeks later. However, median T cell levels were 3.6 times lower in those who had the longer dosage interval (the authors note that the relatively low T cell responses in both cohorts in the study may be because of their age). This again shows how early we are in our understanding of the virus and immunity to it.

Will covid-19 vaccine boosters be necessary?

Albert Bourla, the chief executive of Pfizer, has said that a booster dose will “likely” be required within 12 months of the second dose. However another leading scientist  argues that SARS-CoV-2 is known to mutate relatively slowly, and early studies have found there is still good cross reactivity against new versions of the virus.19 She thinks it is unlikely that the immunity induced by the original vaccines won’t be enough to tackle new variants.

An article published in Science in March 2021 reviewed the evidence so far and concluded that the currently available vaccines give sufficient protection against extant and foreseeable variants.20 “Ultimately, the best defence against emergence of further variants of concern is a rapid, global, vaccination campaign—in concert with other public health measures to block transmission,” the authors concluded. “A virus that cannot transmit and infect others has no chance to mutate.”

“Clamping down [on] this pandemic when we know we have the tools to do so worldwide is our first priority, as opposed to thinking about boosters that may not be needed for rich countries.”