COVID-19: CROSS-IMMUNITY - A GAME-CHANGER?

You may know of the expression QWTAIN, which stands for "Question Where The Answer Is No".  You'll often see a headline in the more 'popular' press, which you realise immediately means the answer is going to be no,  But there's a whole page written about it.  In the 'quality' press, such headlines are banned.

But in this case it is a real question.  Could cross-immunity be a game-changer?  The answer is quite possibly Yes.

But first let's look at vaccines and natural immunity.


VACCINES

Vaccines are seen as a potential game changer, in potentially providing vast numbers of people in the population with immunity from COVID-19.   But most people won't get access to a vaccine until well into next year, 2021, if at all.

Any vaccine has to be both effective and proved safe before a vast proportion of the population can be injected,   You can't have the majority or even a minority of the population being damaged in some way.  Such as if the vaccine were to kick off a 'cytokine storm' like the real infection, which would put a proportion of recipients in hospital.  That risk would be unacceptable, by comparison to the risk of injected people otherwise getting a dose of the real virus.

Russia has just announced that they are going into production of their vaccine before the safety and effectiveness tests have been carried out.  Though to be fair so is production for the Oxford vaccine.  To be ready as soon as safety and effectiveness are confirmed.  If they are.  There is no guarantee. This article sets out what safety and effectiveness work is required.

Testing can't be rushed when it is a matter of seeing what effects occur over a prolonged period of time.  There are acceleration techniques, but basically you just have to wait.



NATURAL IMMUNITY

It is still unclear whether even natural immunity would be gained by having the actual COVID-19 disease.  Some evidence is emerging, though.

Sweden has found that their testing records show nobody has been tested positively twice, here at 20min25 .  Nobody has been re-infected.


But far more evidence than that is needed, especially as to how long immunity might last, or whether a mild infection provides any immunity.

Even with high infection rates, it would take years before a significant proportion of the population would have had COVID-19.  In England, the population is around 55 million.  The current infection rate is estimated to be around 5000 new infections per day.  This is some 5 times the official rate, around 1000, due to various inherent factors such as not everybody with the disease getting tested.

At a steady 5000 a day, it would take 11000 days or some 30 years for everyone to have been infected.  A similar time period applies to any country, with similar infection rates per million.. 

60% - 70% is believed to be enough to cause 'herd immunity', where new infections tend to die out due to uninfected people becoming rarer.  That's still 18 years to reach that lower target at that rate.  Years, not months or weeks.

'Herd immunity' isn't going to happen naturally in any reasonable period of time without some assistance.  Which is why no country, even Sweden, is following herd immunity as its strategy's primary objective.


CROSS-IMMUNITY

In that context, evidence is mounting of cross-immunity.


The Corona Virus Family

There are four other corona viruses that cause symptoms like a common cold.  15% or more of common cold infections are believed to be caused by one of those four types of coronaviruses.  There's also the original SARS and MERS viruses, that are deadly but weren't as infectious as the SARS-COV-2 virus for COVID-19, and so were quickly stamped out.  There are still some survivors of SARS and MERS who are providing some useful scientific feedback.

It may be that someone who has had the snuffles with a corona virus cold will be immune from COVID-19.  A 'group immunity' if you like.


The Science Of Immunology

We hear a lot about antibodies, produced by B cells made in Bone marrow.  A number of studies have shown that antibodies are highly specific to a particular type of corona virus.  My friendly Immunology Professor tells me that "antibodies in people previously infected with common cold corona viruses seem unable to cross-recognise Covid-19 at all."

The cause of cross-immunity appears to be T-cells  the other main component of the immune system   Levels of T-cells that respond to SARS-COV-2 in a laboratory test are being found in around 50% of city populations and elsewhere where common colds are common. But only limited research on T-cells for corona viruses has been done so far.

T-cells are kept by the body (and are continually produced) for many years.  They've been found in survivors from the original SARS virus 17 years later.  Though T-cells tend to reduce in number in older people,  That may explain why the over 50s are been more likely to catch COVID-19, or have more serious symptoms.

Antibody testing has initially dentified around 70% of people in the UK having had COVID-19 without symptoms, known as being asymptomatic.  The Professor also tells me "antibody tests are highly specific, and so are the antibodies. If done properly, there should not be any false positives for antibodies." and "a lot of asymptomatic carriers are real".  Whether that is as much as 70% remains to be confirmed.

This new study looking at T cells, though not yet peer-reviewed, found:

• For people who have been confirmed as having had COVID-19; "Circulating SARS-CoV-2-specific CD8⁺ and CD4⁺ T cells were identified in ∼70% and 100% of COVID-19 convalescent patients, respectively"
•  For those who weren't confirmed as having had COVID-19: "...we detected SARS-CoV-2-reactive CD4⁺ T cells in ∼40%–60% of unexposed individuals, suggesting cross-reactive T cell recognition between circulating “common cold” coronaviruses and SARS-CoV-2."

In the latter case it is unclear whether those individuals could have had COVID-19 asymptomatically.  But the chances are these are people who hadn't had it, but had corona virus common colds.


The Consequence for Herd Immunity

So if around 50% of an area's population have had a coronavirus common cold, and so have relevant T-cells, only 50% would be susceptible to COVID-19.  Then the 60-70% target for herd immunity to COVID-19 is reduced substantially.  Estimates are it would only need around 20-30% if people to have had COVID-19 itself.

That's the sort of level being found in Stockholm, Sweden for example.  That would explain why Sweden's new cases, as in the graph below, have reduced so quickly.  Dr Anders Tegnell, the state epidemiologist was saying recently that he didn't understand why this happened, implying he wasn't aware of cross-immunity.  He certainly wasn’t relying on it for the country's strategy.  So it looks like Sweden has struck lucky!

Sweden's new daily infections to 31 July

WHAT WOULD BE THE CONSEQUENCES OF CROSS-IMMUNITY?

There would be several consequences of cross-immunity:

1. Asymptomatic levels:  The level of asymptomatic carriers of COVID-19 look to be around 70%.  .   .  .                                                                                                                                                                                                                                                                                      
2. Testing for antibodies: A positive antibody test means positive for COVID-19.. Few if any  'false positives'.                                                                                                                                             
3. Testing for T cells:  The tests for T cells used in the lab studies could be scaled up for commercial use                                                                                                                                            
4. Natural Cross-Immunity from Common Colds:  Cross-immunity using T cells could provide natural immunity to those people who have had a common cold due to one of the four other corona viruses                                                                                                                                                                                                                                                                                         
5. Natural Immunity from COVID-19:  If there is cross-immunity, that also suggests that  people who have had COVID-19 will have produced T cells for natural immunity that could last years.  At least in some people.                                                                                                                                            
6. Vaccines: Hopes raised that a vaccine that produces a T-cell response will provide immunity for years rather than weeks                                                                                                                                            
7. Herd Immunity:  Would reduce the threshold for herd immunity; from 60-70% to more like 20-30%.  For places that have had an initially high infection rate, such as Stockholm, that looks like it is already having some affect on reducing infection rates

IN CONCLUSION

Could 'cross-immunity' be a game changer?  Yes, if early indications are supported by subsequent proof.

There are several consequences, as set out above.

It may explain why younger people have been spared serious symptoms, on the whole, if they have previously had a common cold type of corona virus.   Although severe initial symptoms and LongCOVID does happen with younger people, presumably for those who have not had any type of corona virus infection before.

For older people, who tend to lose their T cells, they will tend to lose their immunity and be especially susceptible to COVID-19..

Let's hope that 'herd immunity', where infections peter out, will be reached much earlier than if immunity was relying on COVID-19 alone.  Hopefully exposure to one or more of the other coronaviruses will be helpful in that regard. A 'group immunity'.

But it is perhaps too early to rely on cross-immunity.  Studies are taking place of the science, such as with survivors of SARS, and further evidence is being collected from infection rates in different communities around the world.  We need to wait for further proof.

If cross-immunity happens in England, hopefully we can reduce or eliminate the need for local or national lockdowns.  But for now the experience in the north west, where infection levels remain stubbornly high, suggests we're not near that position yet.

Fingers crossed though!