Research by Oxford University scientists suggests that many people may already have innate resistance or cross-protection against COVID-19 from exposure to seasonal colds.
This innate resistance or cross-protection, which may mean a group of the population is unable to transmit the virus, can greatly reduce the herd immunity threshold (HIT).
It should be noted that the paper, titled “The impact of host resistance on cumulative mortality and the threshold of herd immunity for SARS-CoV-2”, is yet to be peer-reviewed.
There are currently seven coronaviruses that can infect people, of which SARS-CoV-2 (COVID-19) is one.
Four of the seven coronaviruses – HCoV-OC43, HCoV-HKU1, HCoV-229E, and HCoV-NL63 – continually move through the population and produce mild symptoms of the common cold.
Coronaviruses all have crown-like spikes on their surface, which is where their name originates from.
Researchers previously suggested that many people have background immunity to COVID-19, which may include cross-reaction from T-cells that are activated by the common cold.
It is theorised that cross-protection may develop because of the similar structures in all coronavirus, notably the crown-like surface spikes.
The body’s immune system, which has previously been exposed to a common cold coronavirus, may recognise COVID-19 as a threat because it looks similar to a common cold coronavirus.
The body will, therefore, recognise COVID-19 as a threat and activate the immune system against it, even though the person has not been infected by this virus before.
This, in turn, can prevent COVID-19 from making the person sick, or at least make the symptoms less severe.
“T-cell and IgG antibody activity have been reported in non-exposed individuals to SARS-CoV-2, suggesting that resistance to infection may accrue from previous exposure to endemic coronaviruses,” the paper said.
Impact on herd immunity threshold
The study builds on this theory, suggesting that this cross-protection, in combination with innate resistance, can have a big impact on the herd immunity threshold (HIT).
The Oxford scientists said it is widely believed that the HIT required to prevent a resurgence of COVID-19 is in excess of 50% – often quoted as 60% to 70%.
The study demonstrates that HIT may be greatly reduced if a fraction of the population is unable to transmit the virus.
“The drop in HIT is proportional to the fraction of the population resistant only when that fraction is effectively segregated from the general population,” it said.
The paper added that significant reductions in expected mortality can also be observed in settings where a fraction of the population is resistant to infection.
These results help to explain the large degree of regional variation observed in seroprevalence and cumulative deaths.
It further suggests that sufficient herd immunity may already be in place in the UK to substantially mitigate a potential second wave.