This week has seen another spike in Ireland's Covid numbers, with Tuesday's 3,726 positive cases the hightest since January 2021. The Chair of the Irish Epidemiological Modelling Advisory Group, Prof Philip Nolan, has said that there was a 25% increase in the number of cases between last week and this week.

While Ireland's vaccination rate is still high - nearly 89% of the population over the age of 12 were fully vaccinated as of last week - our case numbers remain stubbornly high. This has led to some comparisons with case numbers in other locations, such as New York and Tokyo.

We spoke to Prof Christine Loscher, Professor of Immunology at DCU, to get her take on such comparisons and some reasons for Ireland's current high numbers.

Seasonality

"I think that the one that kind of sticks out the most to me is seasonality and the general seasonality of coronavirus. We haven't had a winter with Delta yet. I've had a look at all of the other countries in Europe that are just coming into the same season as we are and we're seeing the same trend. Actually some of the countries, they're tripling or quadrupling their numbers in the space of just the month of October. Some of them have gone 10 times or 20 times more.

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From RTÉ One's Six One News, NPHET expresses concerns over increased Covid-19 infections

Whereas, if you just look at the weather in New York, they haven't moved indoors yet. But if you look at us compared to other countries in Europe that have a similar kind of season to us, they're all experiencing the same surge literally this week."

Unvaccinated people

"The problem with having a cohort of unvaccinated people for a long time means that the virus gets to do the rounds. If you have 75% of the population vaccinated, and that's a good spread amongst the age groups, the chances of them all mixing at the same time are lower. If you've got a vaccination rate of 75% in people above a certain age, you still have a lot of people unvaccinated mixing and I think that's a real playground for viruses.

"While New York has a similar level of vaccination to us, their vaccination is more spread out amongst the population. What I mean by that is that they rolled out vaccines for 12 to 17 year olds way quicker than anywhere else, which means that they took another kind of unvaccinated population out of the picture way quicker than we did. We were spending a lot of time up until very recently, with a lot of unvaccinated cohorts, up to the age of 18. The uptake has been generally okay for 12 to 16 year olds, but we're not quite there yet."

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From RTÉ 2fm's Jennifer Zamparelli show, Prof Christine Loscher on the possibility of Covid vaccines for 5 to 12 year old children

Levels of natural immunity

"Very early on, Covid ripped through New York. Their population is one and a half times ours, their case numbers have been three times ours. They've had about 1.2 million positive cases, which means they probably have a very high level of natural immunity that we possibly don't have here. In the very early days of recording cases, they weren't testing people: if you weren't sick enough to be in hospital, you weren't tested.

"1.2 million cases means about 13 or 14% of New York population already have had Covid. Not only do they probably have high levels of natural immunity, but those who went on to get vaccinated have super immunity because getting Covid and then getting vaccines actually gives you really, really great immunity so they potentially have more of a herd immunity effect than we have here."

Delta

"We've had a much different infection profile here because of Delta. Look at New York's Delta wave: they never got a peak and then came down and they're peaking again. They kind of went up and they've stayed up, they've been steady. They've probably had more cases for longer than we had; ours were lower and have gone up a bit."

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From RTÉ Six One News, Dr Nuala O'Connor discusses the rise in Covid case numbers

Travel

"One of the things that we've suffered from greatly is our proximity to Northern Ireland and to the UK and the overspill of not just the case numbers, but also travel. There were huge travel restrictions in the United States that we just didn't have for such a long time, either in Ireland or in Europe. I think it'll be interesting to see what happens next in terms of their case numbers now that they've only just opened up their travel.

"It's a really interesting comparison actually between Ireland and New York because it's always an accumulation of different factors that explain things. We can never point to anything singly. I think it's the accumulation of all of those that are potentially keeping their numbers down. The other thing is, it's very hard to find out information on the level of access that they have to testing or how readily available testing is. I would suspect that just like early in the pandemic, that there are potentially people not presenting for testing and they're not being reported."