COVID-19 Diary – Day 208

One of our regular readers requested an update on what we’ve learned about long term immunity to COVID-19 so far. Here is some of the latest data we could find.

COVID-19 Long Term Immunity: No?

Since almost the first days of COVID-19, people have been asking how long a person remains immune after they’ve been infected with the virus. While there is yet to be a single definitive answer (and we may never have one), we know more about the prospects of long term immunity today than we did even 3 or 4 weeks ago.

A recently published study in the Nature Medicine Journal took an interesting approach to the question. Rather than focus on COVID-19, they instead looked at seasonal coronaviruses through donated blood samples (every 3-6 months) over a period of 35 years, hoping to find consistent re-infection behaviour that might shed some light on COVID-19’s possible characteristics. The study only looked at 10 people, but the results they found were relatively consistent both between test subjects as well as over time.

The interval between reinfections determined for the ten individuals. White dots: reinfections without an intermediate decrease in antibody levels; black vertical lines: median reinfection times.

There were a total of 101 infection events recorded from all 10 subjects, ranging from 3 to 17 per individual. The reinfection times ranged between 6 and 105 months but were most frequently observed at 12 months. While we can not be sure that this data will be consistent for COVID-19 as well, it could imply a seasonal nature to the virus.

COVID-19 Long Term Immunity: Yes?

A different study, this one published on August 14th in Cell, theorizes that t-cell based immunity to COVID-19 will be long-lasting, in the term of several years. This theory comes from testing a variety of donated blood from patients who tested positive for COVID-19 early in the pandemic. The blood samples exhibited robust memory t-cell responses months after infection, even in the absence of detectable antibodies specific for SARS-CoV-2.


Further to the above, the study confirmed that the t-cells generated to kill SARS-CoV-2 share many characteristics and defining features with the t-cells generated to kill SARS-CoV-1, and those  T cells have been shown to persist for many years after infection. While this is not a confirmation that t-cells for SARS-CoV-2 will behave the same way, it is encouraging data. (authors note: this study is very technical and I am generalizing heavily in this summary, if you want to know more please read the full text here).

The study also mentioned that they tested cryopreserved blood samples taken before COVID-19 was known to exist in humans. They discovered cross-reactive t-cells, t-cells that were generated to fight something that wasn’t SARS-CoV-2 but still reacted against SARS-CoV-2. They theorize that these t-cells may have been generated to fight a seasonal coronavirus infection, and question if they might offer some level of pre-existing immunity to SARS-CoV-2 infection.

COVID-19 Pre-Existing Immunity?

An article published in the British Medical Journal on Sept 17th asked a very important question: “Do many people have pre-existing immunity?“. Normally this would be a very odd question to ask about a new virus, but there is growing evidence to imply it may not be so simple. At the time this article was written, there were at least 6 studies in existence that documented SARS-CoV-2 reactive T cells in people without exposure to the virus (Studies: 1, 2, 3, 4, 5, 6). These reactive t-cells were found in 20-50% of all samples of non-SARS-CoV-2 exposed blood tested, depending on the study. The blood was sampled from different countries, including the US, UK, Germany, Sweden, and the Netherlands.

Overall this article raises many more questions than it answers, and ultimately it suggests an increased focus on studies on better understanding t-cell immunity. The article does suggest, however, that some of this data could come from phase III vaccine trials, from the placebo wing of the trial.

Halloween in BC

The BC CDC has released its guidelines for a safe Halloween during pandemic times. While they have many specific suggestions, most can be broken down into 3 very clear statements:

  • Skip Halloween parties this year
  • Trick or treating in small groups can be a safe and fun activity
  • Get creative in making space when handing out treats

Many adults and older kids will probably be disappointed (but hopefully not surprised) by the request for parties to be skipped this year. trick or treating in small groups, made up of your safe-6 bubble, should help ensure risks are kept at manageable levels. Many people have already begun working on ways to hand out candy from a distance, and one of the more popular solutions is a candy slide (see below).

That’s all for now, stay safe everyone!

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