杏吧原创

Nearly everyone in Iran has been聽infected by the coronavirus at some point during the covid-19 pandemic, and some have caught the virus more than once, but the country still hasn鈥檛 achieved herd immunity. Instead, Iran is seeing a聽punishing new wave of deaths driven by the delta variant.

Iran was one of the first countries after China to be hit by the pandemic, and it had a slow start to its vaccine roll-out. By July 2021, only about 3 per cent of the population was fully vaccinated against the coronavirus, according to at the University of Oxford, although information from Johns Hopkins University in Maryland shows that proportion has now risen to about 23 per cent.

The Council on Foreign Relations, a US think tank, has 鈥渉alting and ineffective鈥�, and said the country鈥檚 response has been hampered by mixed messages from authorities.

Understanding the effects of covid-19 in Iran has been made more difficult by limited official data. Iran鈥檚 Ministry of Health and聽Medical Education stopped releasing province-level data on confirmed cases and deaths in March 2020. Ghafari also says that low and middle-income countries are more likely to under-report cases and deaths due to a lack of聽health infrastructure and testing capacity.

To get round the shortfall in information, Ghafari and his colleagues analysed figures from Iran鈥檚 National Organization for Civil Registration (NOCR) on how many deaths there were from all causes in the first nine months of 2020. They compared these with historical data to see how many more deaths there were than usual. The researchers used this excess deaths figure as a proxy to estimate the number of covid-19 deaths and population-level exposure to the virus 鈥� an approach that has proved accurate in the UK and South Africa.

The NOCR began releasing past weekly data stratified by age group聽in August, which allowed the researchers to reconstruct the聽dynamics of the pandemic in聽Iran from January 2020 through聽to September 2021.

They calculated how many people in each province had contracted covid-19 using known global figures on the percentage of聽infected people who die, known as the infection fatality rate (IFR) for each age group. Dividing the number of excess deaths by the IFR gives an estimate for the number of infections, and allows you to tell what proportion of the聽population is infected, also known as the viral attack rate.

The analysis showed that the total infected population was probably very high in many provinces. Eleven of them had rates over 100 per cent as of 17 September. The highest rates were seen in Sistan and Baluchestan province, which had an estimated attack rate of 259 per cent. If accurate, that would mean most people have had the virus twice and some for a third time. The researchers conclude that herd immunity through natural infection hasn鈥檛 been attained in Iran in spite of widespread exposure to the SARS-CoV-2 virus, probably because of immunity waning over time, susceptibility to new variants of concern like delta or a combination of the two.

at McMaster University in Canada says studies using direct measurements of聽infections are needed to conclusively determine the attack聽rate of the virus for a given population. However, he finds the聽study鈥檚 conclusions plausible. 鈥淥verall, what鈥檚 happening globally is that it certainly does appear that antibodies wane and聽there鈥檚 not this point where, miraculously, herd immunity or herd effect is achieved,鈥� he says.

Loeb says he isn鈥檛 aware of any聽pathogens for which herd immunity has been reached without vaccination. 鈥淏y far the safest and most effective way to聽achieve herd immunity is through vaccination,鈥� he says.

The first strong indication that herd immunity against covid-19 can鈥檛 be achieved without vaccination came from Manaus in聽Brazil. In October 2020, a study of blood donors there showed that聽76聽per cent of the population had been infected with the coronavirus, which was above the聽theoretical herd immunity threshold of 67 per cent for that community. That threshold is based on limiting the average number of other people an infected person is likely to go on to聽infect聽鈥� the R number 鈥� to three. In聽December and January, in cases, hospitalisations and deaths, which聽was attributed to the highly transmissible gamma variant.

Evidence is piling up that the immunity conferred by infection wanes. A recent study of SARS-CoV-2 and related coronaviruses found that reinfection by SARS-CoV-2 occurred in about 16 months, less than half the time it takes for endemic 鈥渃ommon cold鈥� coronaviruses. And found a covid-19 reinfection rate of 27 per cent during the surge caused by the delta variant earlier this year.

David Fisman at the University of Toronto in Canada says, 鈥淚ran is a big and complicated place and I think the work [these researchers] have done has substantial face validity.鈥� However, he says the study鈥檚 conclusions rest on two big聽assumptions: that all excess mortality is due to covid-19, and聽that the IFR estimates can be聽applied to Iran. He says if the IFR values aren鈥檛 the same there, because of overwhelmed health systems or a new, deadlier viral variant, the calculated attack rate would then also be inaccurate.

The analysis is consistent with previous studies showing that immunity to covid-19 fades and new variants aren鈥檛 stopped as effectively by immune defences, says at Queen Mary University of London. 鈥淭here is no country so far, 19聽months into the pandemic, that has reached herd immunity. And by that I mean, the point at which you lift all restrictions and the pandemic will actually die out because there鈥檚 nobody left to infect.鈥�

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