Public Health England published a press release on 14 January 2021 which  suggested that past COVID-19 infection provides some immunity, but people may  still carry and transmit the virus.  

The study found that past COVID-19 infection provides some immunity for at least  five months, but people still can carry and transmit the virus. 

Studies suggested that those with immunity may still be able to carry the virus in their  nose and throat and have a risk of transmitting to others.  

Public Health England has been regularly testing tens of thousands of healthcare  workers across the United Kingdom since June for new COVID-19 infections as well  as the presence of antibodies which suggest people have been infected before.  

The SIREN study did not look at vaccines and no conclusions could be drawn about  their effectiveness.  

PHE scientists working on the study have concluded naturally acquired immunity as  a result of past infection provides 83% protection against reinfection, compared to  people who have not had the disease before.  

This appears to last at least for five months from first becoming sick.  

This may suggest that those who acquires the infection in the early first wave may  now be vulnerable again.  

Between 18 June and 24 November, scientists detected 44 potential re-infections, two of which were probable and 42 which were possible re-infections out of 6614  participants who were tested positive for antibodies.  

This represents an 83% rate of protection from reinfection.  

Public Health England continued to warn that those with antibodies have some  protection but early evidence from the next stage of study suggests that some of  these individuals still continue to carry high levels of virus and could continue to  transmit the virus to others.  

They suggested that everyone must continued to follow the rules and follow  government guidelines, especially within this lockdown. 

The SIREN study performed regular antibody and PCR testing on 20,700 including  frontline clinical staff and those in non-clinical roles, from 102 NHS trusts from June  6; 6614 of these participants tested positive for COVID-19 antibodies when recruited.  

Of the 44 potential re-infections identified by the study, two were designated probable  and two possible based on the amount of confirmatory evidence available.  

If all 44 cases were confirmed, it would represent an 83% rate of protection from  reinfection, while if only the two probable re-infections were confirmed, the rate would be 99%.  

Research is clearly required.  

The study found that antibody protection after infection lasts for at least five months,  on average, and scientists are currently studying whether protection may last longer.  

The two probable re-infections reported having experienced COVID-19 symptoms  during the first wave of the pandemic but were not tested at the time.  

Both patients reported that their symptoms were less severe and second time.  

None of the 44 potential infection cases were PCR tested during the first wave, but  all tested positive for COVID-19 antibodies at the point of recruitment to the study.  

The study continues and will follow participants for 12 months to explore how long  any immunity may last, the effectiveness of vaccines and to what extent people with  immunity are able to carry and transmit the virus. 

Dr Paul Ettlinger 
BM, DRCOG, FRCGP, FRIPH, DOccMed

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