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expert reaction to media stories on app pings and self-isolation, plus new ONS data on self-isolation after being in contact with a positive case in England

There have been several media reports on the high number of isolation alerts from the NHS Covid-19 app, and new data has been published from the Office for National Statistics (ONS) looking at the behaviour of individuals required to self-isolate after being in contact with a positive case of COVID-19.


Dr Penny Ward, Faculty of Pharmaceutical Medicine, Visiting Professor in Pharmaceutical Medicine, Kings College London, said:

“Completion of vaccination reduces, but does not completely eliminate risk of infection if an individual is in close contact with an infected person.  In addition, although vaccination reduces viral load and reduces potential for transmission of infection, it does not completely eliminate the risk of transmitting infection to a close contact.  Hence policy on isolating if asked should not be made on the basis of vaccination alone but also take into account the risk of COVID among the people that the individual concerned may be in close contact with during the period they might be incubating the infection.”


Dr Jonathan Stoye, Group Leader, Retrovirus-Host Interactions Laboratory, The Francis Crick Institute, said:

“Being double jabbed brings relative but not absolute protection from SARS-CoV-2 and does not eliminate the possibility of infection following contact.  However for people who have had two doses of vaccine (with the second more than two weeks before) and who have a negative PCR (not just lateral flow) test administered a few days after the ping-contact, it would not seem unreasonable for it to be considered whether the self-isolation instruction needs to apply to those people, especially since this is supposed to become official policy from August 16th.”


Dr Peter English, Retired Consultant in Communicable Disease Control, Former Editor of Vaccines in Practice, Immediate past Chair of the BMA Public Health Medicine Committee, said:

“Case rates are rising dramatically; and so far, we have failed to stop spread by means of vaccination alone.  This may be possible eventually; but probably only once a high proportion of young people (12-30) have been vaccinated: this age group is highly social, and therefore contributes considerably to transmission.

“Many people do not know they are infected and infectious.  Some have mild symptoms; some none at all.  And many people who are infected acquire their infection from people with few or no symptoms.  Vaccination reduces the likelihood of being infected, and, if you are infected despite being vaccinated, of being infectious.  Note – it reduces the likelihood; but it does not eliminate it.  The delta variant is much more infectious than previous delta strains; and, whereas with the original or alpha variants, a single dose of vaccine was moderately effective at preventing infection and transmission, full vaccination (at least two weeks from the second dose) is required for the same efficacy against the delta strain.

“This means that we have to continue to rely on systems to identify people who may have been infected, and to require them to self-isolate until we are sure they are not going to infect others.  With very high transmission rates it is inevitable that there will be many such people; but if we relax the requirements to self-isolate when pinged, transmission rates will inevitably increase, with even greater consequences.

“We are watching the efficacy of vaccination in reducing onward transmission.  It takes quite a long time to acquire enough data to be sure how effective vaccines are and will be at this.  We all hope that it will turn out that full vaccination is sufficiently effective – that the odds of transmission from somebody who has been fully vaccinated will be low enough to drop the requirement to isolate.  (An intermediate efficacy requirement may allow normal social activities, but e.g. not to work with vulnerable people.)  Until we have enough data – and we don’t yet – it would be too risky a gamble to relax the requirements to self-isolate when “pinged”.

“With regard to people who are pinged but test negative: it can take 2-3 days from exposure to the tests being able to detect the virus; and home tests are relatively low in sensitivity (i.e. there are many false negatives – people who ARE infectious, but in whom the test is falsely “negative”).  At population level, this has to be converted into probabilities.  What is the probability that you were genuinely at risk of being exposed?  What is the probability that you will have been infected (which varies with vaccination and prior infection status)?  What is the probability that, if infected, you will be infectious to others – which again is influenced by vaccination and prior infection status?  And what is the probability of being infectious if the test result is negative?  There are a lot of variable in there, and these are what the experts on SAGE will be considering.  Given the rapidly rising case rates, we need to be cautious; but it is possible that they will conclude that being fully vaccinated AND testing negative reduces the risk of being infectious to others to a low enough level to be acceptable.

“(Caveat – this assumes that the app system works properly, which is not my area of expertise.)”


Prof Jon Crowcroft FRS FREng, Marconi Professor of Communications Systems in the Computer Laboratory, University of Cambridge, said:

“The number of notifications is exactly in line with what you would expect given 250,000 positive tested people in the last week alone – if each person testing positive met about four people a day and you had a roughly 50/50 chance they and the other person were running the app (which is approximately right) then you’d expect to see about 500,000 people getting notified.

“There’s nothing much wrong with the app – just people aren’t socially distancing enough.”


Dr Simon Clarke, Associate Professor in Cellular Microbiology at the University of Reading, said:

“While these conclusions (in the ONS data on self-isolation after being in contact with a positive case) are based on a relatively small sample size, the results show that self-isolation for contacts of confirmed cases is still a very effective tool for reducing transmission of the virus and keeping R rates down.

“The fact that around one-third of people who are asked to self-isolate as contacts go on to become ill themselves shows the importance of this system.  This proportion has remained consistent in recent weeks, despite the rising number of total cases and people isolating as contacts of cases.

“If you are asked to self-isolate as a contact of someone who has tested positive you should most certainly heed the warnings.  From a scientific perspective, we know that this virus, particularly the now more dominant variants of concern, has become considerably more transmissible.

“Because we are in a rapidly moving situation with vaccinations, the evidence on the impacts of being double jabbed are still emerging.  But people should remember that being double jabbed, while providing a good level of protection, is still not 100 per cent.  It is still perfectly possible to be double jabbed and catch the virus.

“We should also remember that some older people or those in vulnerable groups who had their jabs many months ago may now have waning immunity.

“It is a great inconvenience to be asked to stay at home for 10 days, but it is much, much more damaging for people’s health and the economy to have a rampant new wave of infections ballooning out of control.  Self-isolation is one of the most effective tools we have to prevent spread.  As we reopen the economy and society fully, and change rules on home working, mask wearing and social distancing, it is one of the few effective tools we have left.”



All our previous output on this subject can be seen at this weblink:



Declared interests

Dr Peter English: “Dr English is on the editorial board of Vaccines Today, which is an unpaid, voluntary, position.”

None others received.

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