The question I get more than any other:
I had Covid. I had an antibody test to prove it. Am I protected (and do I need to get the vaccine)?
Let me start with the usual disclaimer: THIS IS NOT MEDICAL ADVICE. I AM NOT A PHYSICIAN.
But the answer is now increasingly clear: natural immunity from Covid following infection and recovery is HIGHLY protective against future Covid infections. Rates of reinfection are very low.
Perhaps natural immunity eventually wanes, but we don’t know when. In fact, a little-noticed paper from June suggests it may actually strengthen for at least a year - and provide plenty of protection from Delta and other variants.
I am not going into the problems with vaccine-generated protection today or with our political unwillingness to recognize natural immunity. (Remember, GOOD NEWS - we could all use it).
Let’s just look instead at why natural immunity works so well.
You immune response comes in two forms: “humoral” and “cellular.”
When you are infected with Sars-Cov-2, your body’s “B-cells” - part of the immune system - quickly pour out “antibodies.” These antibodies attack the viral particles circulating in your blood and other fluids, hoping to keep the virus from entering your cells and replicating itself.
This is humoral immunity. Your B-cells make antibodies in many different shapes. Some are better at sticking to the virus. Scientists call these “neutralizing” antibodies because they neutralize the “antigen,” the foreign body attacking you, keeping it from entering your cells.
Amazingly, your B-cells quickly figure out which antibodies neutralize most effectively and make more of them, while cutting back on those that don’t work.
At the same time, another part of the immune system - killer or CD8 T-cells - attacks cells that the virus has already infected. You destroy your own cells to prevent the virus from using them to make more copies of itself. This is cellular immunity.
For a few days after you are infected, your immune system is in a race with the virus. If you win the race, defeat the virus, and recover - as the vast majority of people infected with Sars-Cov-2 do - within a week or two you should have no measurable levels of virus in your body.
With no invader provoking a response, your B-cells will stop making antibodies against the virus. Over time, the antibodies you have made will slowly degrade - a change scientists can measure.
Antibody levels are measured as “titers” - how much the part of the blood that contains antibodies must be diluted before it stops neutralizing the coronavirus (or any antigen).
The details of how scientists measure titers are complicated. But the takeaway is simple - higher titers mean more antibodies in the initial sample. Higher titers are better, unless they are so high they indicate an overactive immune response. Over time, titers will decline.
But your immune system has not forgotten the virus. Both B-cells and T-cells are now trained to respond to it, should it reappear. (Confusingly, the immune system includes two types of T-cells, CD4 and CD8, and many subtypes of both B- and T-cells.)
This “adaptive” response explains why people remain immune to some viruses for their entire lives after being infected once - or inoculated with an effective vaccine.
Scientists have different ways to measure your immune response.
They can measure how tightly your antibodies bind to an antigen (the virus), how many different parts of the antigen they attack, how quickly your B- and T-cells will ramp up to make new antibodies or attack infected cells, and even whether those antibodies can recognize new variants of the original antigen.
In June, Rockefeller University researchers published a paper in Nature examining how antibodies changed up to a year after coronavirus infection and recovery.
The authors found that even “in the absence of vaccination, antibody reactivity to the receptor binding domain (RBD) of SARS-Cov-2, neutralizing activity, and the number of RBD-specific memory B-cells remain relatively stable between 6 to 12 months after infection.” (They also found that vaccination of previously infected and recovered people could boost those responses further.)
However, in a second paper published in late July, the authors found that vaccines did NOT produce a similarly powerful response in the B-cells of people who had not previously been infected. (But we’re not going to talk about that today. Good news, remember?)
In July, another group of researchers published similar findings in Cell:
Again, the authors offered reassuring findings:
Here, we evaluate 254 COVID-19 patients longitudinally up to 8 months and find durable broad-based immune responses. SARS-CoV-2 spike binding and neutralizing antibodies [have] an extended half-life of >200 days suggesting the generation of longer-lived plasma cells… Taken together, these results suggest that broad and effective immunity may persist long-term in recovered COVID-19 patients.
T-cell immunity was also durable, the authors found, and the killer T-cells could recognize not just the coronavirus’s spike protein but other parts of the virus.
(Again, the paper offered reasons to believe vaccine immunity was inferior, but we won’t talk about that, not today.)
A paper from Israel released Sunday reported that antibody titers in people who had recovered from infection fell only 5 percent a month (titers in the vaccinated fell 40 percent a month, though from a higher baseline).
Even better, between four and nine months after infection, the percentage of people with low titers remained roughly flat, at 10 percent.
All these papers should increase our confidence that natural immunity is durable and powerful. But what about real-world data?
Fortunately, we now have several studies examining large groups of previously infected people. In perhaps the most striking, the Cleveland Clinic reported in June a reinfection rate of zero among 1,359 people with natural immunity:
Not one of the 1359 previously infected subjects who remained unvaccinated had a SARS-CoV-2 infection over the duration of the study.
(The political pressure for vaccination is such that the Cleveland Clinic then issued a press release basically pretending the study didn’t say what it did. But we won’t talk about that today, good news, etc.)
And this week, Israeli researchers looking at a much larger group of people also found very, very low reinfection rates. Only 19 out of 16,215 people with natural immunity had a second infection from June 1 to August 14 (far fewer than the number of vaccinated people who were infected in a matched group).
These findings are particularly important because the Delta variant accounted for nearly all coronavirus infections in Israel by this summer. In other words, natural immunity remained protective against Delta.
So there’s your good news. Both the theoretical bench work and the real-world data suggest that natural immunity is long-lasting and protective even against Sars-Cov-2 variants that can evade vaccines.
I’ll leave it to you to decide whether that means you need a vaccine if you have already had and recovered from Covid.
NOTE/CORRECTION: A previous version of this story did not explain that a Rockefeller University study from June had shown that vaccination further increased the immune response in some previously infected individuals (while a later paper showed vaccination produced a much weaker response on those who had not previously been infected).