What is the best way to measure rates of covid immunity?

Globe with face mask during covid pandemic

In my previous post on the covid pandemic I mentioned that the body’s main defence against viruses is T-cells, not antibodies, and that the only reason we test for antibodies instead in clinicial practice is because it is easier and cheaper. I also ventured a hypothesis that the levels of population immunity are much higher than is being found in the antibody tests, and that this is because lots of people who don’t have antibodies do have covid specific T-cells. It turns out that this hypothesis is supported by new evidence.

A study carried out at Karolinska Institutet (where I went to medical school), which is still awaiting publication, looked at the presence of both antibody-based and T-cell specific immunity to covid among people in Stockholm. The data was collected during May. The first covid fatality in Sweden was in mid-March, so at that point covid had been raging for about two months.

The study was funded by Karolinska Institutet, the Swedish Research Council, and a number of private foundations and charities. The authors reported no conflicts of interest.

Study participants were recruited in to five distinct cohorts, with a total of around 200 individuals:

The first cohort was made up of patients who had had a mild infection and recovered. Most of these (78%) had not been sick enough to be admitted to hospital. The few that had been admitted had at most required one liter of supplemental oxygen. This was the “mild convalescent” cohort.

The second cohort was the “severe convalescent” cohort, which consisted of patients who had been sick enough to require larger amounts of oxygen and/or mechanical ventilation, and then recovered.

The third cohort was made up of family members of people in the mild convalescent and severe convalescent group. In order to qualify to be part of this cohort, participants had to have spent time in the same household as the sick family member when they were sick, but not themselves been diagnosed with covid 19. This was the “exposed family members” cohort.

The fourth cohort consisted of a random sample of people who donated blood in May 2020, while the fifth cohort consisted of a random sample of people who donated blood in July to September of 2019. The fifth cohort served as a form of control group, since the blood was collected before the beginning of the pandemic.

Although each of the cohorts is a bit small for my liking, it is an interesting mix, which can potentially answer some important questions about how the immune system reacts to covid, and how prevalent covid was at this point in time in Stockholm.

Now to the interesting part, the results:

Let’s start with the blood donors who donated blood in 2019. They weren’t tested for antibodies (strange, I would have tested them just to have a baseline, to see what the rate of false positives was, but maybe there was some technical reason as to why this wasn’t possible) but they were tested for covid-specific T-cells. Not surprisingly, no-one in that group had T-cells (0/37).

Next, we can look at the people who were convalescing from severe disease. Among these individuals, 100% had antibodies, and 100% had T-cells (23/23). This makes sense. When you have a severe illness, you get a strong immune response.

Next up, let’s look at the people who were convalescing from mild disease. In this group 87% had antibodies (27/31), while 97% had T-cells (30/31). Again, this makes sense – if you have symptomatic disease, then that is a sign your immune system has realized there is an infection going on, so there should eventually be signs of that in the form of measurable antibodies and/or T-cells.

Now we can look at the exposed family members. Remember, this was a group of people that had not shown signs of symptomatic disease. In this group, 60% had antibodies (17/28), while 93% had T-cells! (26/28). This is pretty astonishing, and it shows two things. Firstly, if you lived with someone who had covid then you were most likely also infected. This is true even if you didn’t have symptoms, and even though you didn’t have symptoms, you most likely developed an adaptive immune response. Secondly, that immune response involved T-cells more often than antibodies.

Finally, we can look at the people who donated blood in May 2020. This was a random sample, so we don’t know how many had had symptomatic disease and how many had been completely asymptomatic up to the time point when their blood was drawn. In this group, 13% had antibodies to covid (4/31) and 29% had T-cells (9/31). That is pretty astounding. Now, again, this was a small trial and the cohort was small so the confidence intervals are wide. But it is still remarkable that twice as many people had T-cells as had antibodies in the random sample.

Additionally, this was in May, two months in to the pandemic. If 29% of Stockholm’s population had T-cells in May, then it’s reasonable to expect that by now, three months later, that number has at least doubled, which makes for a very reasonable explanation as to why the covid death rate has fallen so precipitously in Sweden – we’ve now reached the point where we have herd immunity. This is a speculative conclusion considering the small sample size, but it makes sense.

As yet, no proper studies have been performed, to my knowledge, showing that antibodies give immunity to covid, and no studies have been done showing that T-cells give immunity to covid either for that matter. However, we know from experience of infectious diseases in general that an antibody response and/or a T-cell response usually means that you are protected from future infection, at least for a time, and often for a lifetime.

This study has its weaknesses. The main problem with it is that it is small. It would be beneficial if a similar larger study could be performed to corroborate the results. Another problem is that it hasn’t gone through peer-review yet. It is noticeably rough around the edges. The text is overly technical and quite disorganized, and the graphs are hard to understand if you don’t spend a significant amount of time with them. Additionally, there seem to be some bugs in the text, like the number of participants appearing to differ in different places without any explanation offered.

However, the implications of this study are huge.

Firstly, the fact that significantly more participants had T-cells than had antibodies, suggests that if we want to know the true rate of immunity in a given population, then we should be looking at the proportion who have covid specific T-cells, not the proportion who have antibodies.

Secondly, if it’s true that Sweden now has herd immunity, then it seems likely that many other countries will follow over the coming months. By the time a vaccine appears at some point next year (with luck), there might not be many people left that actually need it.

UPDATE 16th August 2020: The study has now been published in Cell.

If you want to read more of what I’ve written on covid-19, check out my analysis of hydroxychloroquine as a treatment. You might also be interested in watching the interview I did with Sky News.

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39 thoughts on “What is the best way to measure rates of covid immunity?”

  1. Glad I was lead to your site. Here in the USA it seems all the information we get has a political slant to it, either to the left or the right. Looking forward to future posts.

  2. Dear dr. Rushworth, wonderful article. I hope that people will repeat the study with larger numbers. Until then, the study design looks good to me. I learned that any sample above twenty people was more than enough. I ran a quick chi square test of your numbers for 17 of 28 vs 26 of 28 and got p=0.004. Thanks for your work, ken

    1. As I understand it from close reading of the article (although it isn’t expressed clearly in the text), everyone who had an antibody response also had a T-cell response. Logically, this makes sense, because T-helper cells need to be activated first before B-cells (which produce antibodies) can be activated.

  3. My interpretation is Sweden’s approach has resulted in a more predictable curve, not the lumps and spikes the locked down are condemned to. Thank you !

  4. I found this to be fascinating! My 21 year old daughter who works in a hospital got Covid in mid May and had unusual symptoms. I took care of her without caution because the symptoms did not seem to be Covid. We later realized when we were both tested for anti-bodies that she was positive and I have had two negative anti-body tests. My husband and other daughter also never got sick. They have not been tested for anti-bodies. I wondered how I could have cared for her without being cautious and did not get Covid. Perhaps this holds the answer!

  5. Like millions of people around the world, I’ve followed news about the pandemic since inception. Not a single article in the US has made mention of the fact that Covid-specific t-cells are a much more accurate way to measure infection rate and ultimately immunity. Instead, we are drowning in stories preaching the religion of lockdown and social distancing, with little empirical evidence to back it up. At least 45% of US Covid deaths have occurred in Long Term Care facilities. In some states, the LTC mortality rate is well over 50%. 50% of all US deaths have occurred in a 300 mile radius of NYC despite the fact that well under 20% of the US population lives in this zone.. Despite a high rate of infection (and rising) outside of this hot zone, the mortality rate is far lower. Clearly, in the US, the hospital/LTC facility response was an exponentially bigger factor in the mortality rate than social distancing and mask wearing.

  6. According to the Australian Government owned national (radio) broadcaster’s trotted-out ‘Expert’ this morning, the RT- PCR test is better than 99.9% specific and deadly accurate. Indeed, it is the “Gold Standard” . Not only but also, said expert corrects the test’s inventor and a talk-back listener, over the notion the test was not designed to ‘diagnose’.
    Once again, with a swab up your nostril and down the throat, we lead the world in quality testing for COVID… (Tongue firmly in cheek)

    Yes, the State of Victoria is in ever-tightening lockdown, and infection / mortality is not improving…

  7. Awesome posts Doc… they explain very well what we are seeing in places like Stockholm, NY and now Florida, Texas, California where once the antibody tests reach about 15% the virus seems to stop spreading. Finally, your subscription button seems to be broken. I tried to subscribe using multiple different email addresses and I get an error saying “please enter a valid email address”. Thanks!

  8. Why if we took a cowpox approach to covid? Are there any coronaviruses that are similar enough to SARS-2 that produce exclusively mild infections but would eventually create long-term immunity? Then we could use them as a sort of vaccination on the cheap.

  9. The real disruption in the health industry will be massive. Metadata technology already shows that reductionistic/dualistic understanding/approaches to the human organism is a “scam”. The biomedical model as unsafe/useless. Health industries will have to learn and change focus to the importance/impact of how feeling get imprinted in the organism(rules everything)


  10. Thanks so much for your scientifically objective article. I am a biologist, who also worked in a virus lab as a technician many years ago, and I have been dismayed by the unscientific approach taken by the US government officials and our now very partisan media. Who seem to decide on a prescription first, that is half politically motivated due to its socio-political side effects, and then cherry pick data and try to socially shame-coerce citizens into doing things that simply delay the inevitable of most everyone being exposed. One major curiously inconsistent thing is that our 1% are perfectly willing to lockdown and destroy the livelyhoods of 10’s of millions of common Americans. But at the same time our leaders are adamant about keeping our borders open and accuse you of racism if you suggest for example that travel and trade originating in China should be either stopped or strictly controlled and carefully examined. At least until China stops its many, known for decades disease producing activities which result in the emergence of another pathogen every few years. While most recently a lead article in the NY Times insulted the nation’s intelligence by calling for a 8 week total US lockdown to ‘defeat COVID by October’. When any thinking person knows that in October after a lockdown that completely destroys our economy, the nation will quickly be reinfected by people getting off planes from other continents and legally and illegally crossing our southern border. And so the remaining people without immunity will get sick and we will have many “spikes” that the media can again shriek about in sensational headlines.

    1. Great article, pity NZ hasn’t taken this into consideration. We are so fantastic, or so were told by the world that ‘Cindy’ is so wonderful as a leader. However she’s not so hot economically. She forgets charity doesn’t start at home/homeless, beneficiaries, it starts with business, tax payers. All beneficiaries say thanx Government, NOT govt money it’s us the tax payers money.🍦😪😪

  11. Indeed, T-Cell response needs to be considered. I’d just give a caveat that we don’t know how long – and how – effective of “immunity” that translates to. Though it needs to be considered.
    Sadly, there are many long-term adverse effects for so many who’ve acquired and supposedly “defeated” the disease. Including my 19 year old son, with no comorbidities. Which is a counter-argument to letting it burn through and people to simply develop their natural defenses. There is a health cost to taking that tack.

    1. You are assuming that 100% of people are susceptible to the disease. This is an incorrect assumption.

      You are also assuming that the presence of antibodies is necessary to show immunity. This is also an incorrect assumption.

      I recommend re-reading the COVID commentaries on this site to achieve a better understanding of the situation.

      1. I don’t think you understand my point. For over 57% to have antibodies in India, at least 57% must have had the disease. Therefore there is not much room for immunity which is not detected by antibodies. If that were the case, the amount of infected in Mumbai would come too close to, or over 100%.

        Secondly, when 57% has antibodies in Mumbai, Sweden has a long way to go to reach the same level of antibodies.

    2. You do realize that your link stated that it was 57 percent in the *slum areas* in Mumbai where people live on top of each other and share public bathrooms, with 16 percent in the population outside the slums? The effective spread is likely much higher in the slum, and it is likely that pretty much everybody is exposed there and the antibodies undetstate the fact. That does not mean that threshold for herd immunity is as high in a richer European society…

      1. Johan, at least we know with certainty that the level of immunity not detected by antibody tests is clearly under 50%, in other case over 100% would have had the disease in Mumbai. We don’t know how many has had the disease, but people are still getting sick there.

        You may excuse me my Finnish perspective, but I don’t get the benefit of having the virus spreading in the society, when we can chose to end it like we have done in most of the Nordic countries and New Zealand.

        First the death toll is in my opinion too high to accept. Over 20 times more have died in Sweden than in Finland.

        Secondly is the economy, which in my option is a secondary issue, but it has anyway turned out to be significantly more expensive to have the virus floating around than ending it. The economy in Sweden has tanked 8.6% last quarter, when the same number is 3.2% in Finland. This is according to experts because of the spreading of the corona virus.


        “Enligt Andreas Wallström, tillförordnad chefsekonom på Swedbank, är den högre smittspridningen en av förklaringarna till den större nedgången.”

        Last, but not least, reaching herd immunity would not anyway stop the spreading of the virus, which Anders Tegnell points out in the following article:


        “Under presskonferensen på onsdagen sa Anders Tegnell att det finns en ”myt” om total flockimmunitet, och att det i stället handlar om en immunitet som håller ner smittspridningen.

        – En immunitet som helt stoppar smittan finns det ingen anledning att tro att det kommer att hända.

        – Det kommer aldrig vara så att den här smittan försvinner, den typen av flockimmunitet kan man egentligen bara uppnå om man kombinerar det med ett vaccin.”

    3. Is testing free in India? No backlogs? Anyone without any regards to education/social status can get tested any minute? Or 57% are only those people that have means have tests? My family is from an underdeveloped country, and believe people there do not go get tested because of those economical reasons. You might only see results of those privileged people.

      1. Testing is not free in India, but those who made the investigation paid the tests.

        However, the result shows that it is not enough to have 20% with antibodies, like in Stockholm/Sweden, in order to have herd immunity. You need a much higher level of immunity (detected by the antibody test).

        Results from Bergamo in Italy also shows the same thing. There over 50% of the inhabitants have antibodies:

      2. With herd immunity, I meant a herd immunity that completely stops the spreading of the disease. It may not be possible to achieve without vaccines according to Tegnell. He talk about herd immunity only as something that slows down the spreading.

  12. Not a member of the medical field but always looking for new information. It is my understanding, and please correct me if I am wrong, that many of the Swedish cases may be from New Swedes that may have never received vaccinations and medical care and may potentially be more susceptive?
    Also, those that donated blood and had no antibodies/Tcells present; it would be interesting to know of their current situation, if they have been exposed.
    Is there any consistency of any particular blood group or otherwise that does not appear to be affected even after exposure?

      1. Thanks for important news Sebastian.
        You may also find this article interesting: ahttps://lynnemctaggart.com/just-do-the-math/?utm_campaign=&utm_content=Just+do+the+math&utm_medium=email&utm_source=lynmctaggart

  13. Dear Dr. Rushworth,

    Thank you for these 2 very enlightening clearly written articles. They answer many of my questions.
    I have more question: what is an asymptomatic carrier, and if it means someone that has contracted covid, but has absolutely no symtoms, how long is that person asymtomatic and could transmit the disease? Supposedly without sneezing or coughing, as they have no symtoms.
    And does herd immunity mean that immunity is passed, kind of, from person-to-person through the air, without us ever knowing that we got something?
    Sorry it sounds so banal.
    Thank you💖

    1. Hi Sibbi,
      An asymptomatic carrier is someone who has the infection but isn’t showing any symptoms. With covid (and many other infections) it is likely that this is the stage where you are most infectious, since it has been found that the moment you start to show symptoms, the level of virus you are secreting drops dramatically (which makes sense, since symptoms are a sign that your immune system is fighting the infection).
      Herd immunity simply means that a big enough proportion of the population has become immune that it is difficult for the virus to find new hosts to infect.

      1. Dear Dr. Rushworth, Thank you 💖 for your answer. Now I have more questions:
        If an asymptomatic carrier is more contagious, but is not sneezing and not coughing isn’t this contagiousness more contained to the close proximity to that person? E.g. transmiting only through breath and handshakes etc.?

        How long time does an asymptomatic carrier stay asymptomatic? I suspect that some asymptomatic carriers never get sick?

        In order for us to get herd immunity are we all going through the asymptomatic carrier stage? How does herd immunity work?

        Thank you 💖 again so much!!!!!
        For your very clear words.
        I have confidence in our body again, that the immune system works beautifully. All this panicky and fear making talk out there is just not making sense.

      2. Coughing and sneezing definitely increase spread of the virus. I’m not sure if there is any research that clearly says whether an asymptomatic person who was infected four days ago or someone who was infected five days ago and is now coughing is more infectious. Studies have shown that viral load drops dramatically when symptoms start.

        A lot of people become infected and never show any symptoms, they just clear the infection without ever noticing it. On average it takes five days from being infected to showing symptoms.

        Not everyone needs to develop immunity for herd immunity to work, just enough people that the virus has trouble finding new hosts. How big a proportion that needs to be immune for herd immunity to develop varies with different infections, and I don’t think anyone knows what that number is for covid yet.

  14. Hello Dr. Rushmore,
    On April 2, about 2 weeks after the economy in the U.S. was shutdown, I wrote a short blog about Covid-19. I wondered: WHAT is happening!
    Towards the end, I mentioned that one of the things that really bothered me was the way the 24/7 news was presenting the news. Everyday, they talked about how many more were sick, how many were dying—rarely mentioning (almost never) how many had recovered. We were doused with fear, fear, and more fear. No hope.
    Now, here it is August.
    I thought life would have gone back to normal by now, but no. Everyday, the news reports continue to give the number of new cases and deaths–without providing all the rest of the statistics to put things into perspective. To top it all off, it appears that now the virus has become part of a political battle here in the U.S. And on and on and on it goes…..
    It is very nice to read your posts that are actually viewing Covid-19 in a rational and informative way. Thank you so much. My April post was just a reaction to the sudden and abrupt changes….blindsided!

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