The latest estimates of Covid-19 infections in the UK are likely to have been influenced by ongoing postal strikes across the country, experts said.
Infections are thought to have increased in England and Northern Ireland in late November, while the trend is unclear in Scotland and Wales.
But there is greater uncertainty than usual in the estimates, as well as a longer time lag in reporting the data, according to the Office for National Statistics (ONS).
This is due to the low number of Covid-19 test results received by the ONS, ‘probably’ following industrial action by postal workers.
“For all UK countries, some results may have increased uncertainty and be subject to change as more test results are received for this period,” the ONS said.
The total number of people in private households in the UK who tested positive for the coronavirus was 1.1 million in the period 17-26 November.
This is up from 972,400 in the week to November 15th.
During the previous wave of infections in early autumn, the total peaked at just over two million in mid-October.
This was well below the peaks seen earlier in the year, when infections rose to nearly four million in July and just under five million in March.
Michelle Bowen, head of health surveillance at the ONS, said: “Infections have continued to rise across England, they have increased in Northern Ireland for the first time in a month and the trend across all other countries in the UK is now uncertain.
“Across all English regions and ages, it’s a mixed picture of uncertainty and rising infection rates, and we will continue to monitor the data closely over the winter months.”
The ONS Infection Survey is the most reliable measure of coronavirus prevalence and is based on a swab sample of households across the UK.
But because the amount of test results received by the ONS is lower than usual, the new estimates cover different periods in the four nations and should be treated with caution.
The number of people who tested positive for the coronavirus in England in the seven days to November 26 was 941,700, or about one in 60 people, compared with 873,200 in the week to November 21, which was also one in 60.
In Northern Ireland, around 36,700 people contracted Covid-19 in the seven days to 23 November, or one in 50, compared with 28,900 in the week to 21 November, or one in 65.
Scotland is likely to have had 88,500 people with coronavirus in the week to 24 November, or one in 60, compared to 91,100 in the week to 21 November, which was again one in 60.
In Wales, 43,400 people were estimated to have contracted Covid-19 in the week to 24 November, or one in 70, compared with 39,600 in the week to 22 November, or one in 75.
Infection rates in England at the end of November were estimated to be highest among school-age children, with 2.2% likely to have the virus, the equivalent of one in 60.
More recent data suggests that the number of people admitted to hospital in England with Covid-19 is on the rise, with 5,501 registered on 7 December, an 11% jump on the previous week and the highest since 10 November.
Admissions rates in England stood at 5.5 per 100,000 people in the seven days to December 4, up week on week from 4.9.
Dr Jamie Lopez Bernal, consultant immunization epidemiologist at the UK Health Security Agency (UKHSA), said: ‘Over the winter, we would expect to see increased activity of Covid and other winter viruses as people they mix again more indoors.
“Covid hospitalizations are highest in older age groups, so it is particularly important that all those eligible continue to come forward to accept their booster vaccine.
“Although Covid-19 and influenza may be mild infections for many, we must not forget that they can cause serious illness or even death for the most vulnerable people in our communities.”
Around four in five (80.6%) people aged 80 or over in England have received a new booster dose of the Covid-19 vaccine, along with a similar proportion of people aged 75 to 79 ( 81.0%) and between 70 and 74 years-old (77.8%), show UKHSA data.
All people aged 50 and over can book an appointment for a booster if they had their last vaccination at least three months ago.
The doses are also available for frontline health and care workers, pregnant women and people with weakened immune systems.