We have observed a significant change in the circulating HAdV serotypes in the GTA of Ontario during December 2008 to April 2010. Of note is a significant decline in HAdV-3 and an increase in HAdV-2 in the population during the study period. These findings demonstrate that multiple serotypes would need to be included in order to create an effective HAdV vaccine. PHOL-Toronto is responsible for testing most, but not all cases of HAdV in the GTA; there are other tertiary institutions that test their own samples, and do not send them to PHOL. Thus, our results are not fully representative of all of the GTA’s HAdV cases. In addition, the generalizability of this study was further limited due to the fact that we were only able to sequence 90 (39%) of the 229 HAdV isolates that were detected at PHOL-Toronto during the study period, since only samples that were tested by viral culture were included. PHOL does not perform culture on samples from outbreaks or ICU patients, as these are tested by multiplex molecular methods. Thus, our sample was biased towards patients with less severe infections.
Yeung et al. showed that the prevalent serotypes in Ontario from September 2007 till June 2008 were HAdV-3 (44, 46%), HAdV-2 (25, 26%), HAdV-1 (17, 18%) and HAdV-21 (5, 5%) . Our study demonstrates a similar serotype distribution with HAdV-3 (34, 38%), HAdV-2 (30, 33%), HAdV-1 (14, 16%) and HAdV-21 (6, 7%) also being most prevalent. We also documented a significant decline in HAdV-3 infections in the final 4 months of the study, suggesting a change in serotype distribution. Chen et al. also documented a marked shift in circulating serotypes in consecutive seasons in hospitalized children in southern Taiwan between 2001 and 2002 . In 2001, HAdV-4 was found in the majority (57%) of isolates, while HAdV-3 was rare (5%) . In 2002, while HAdV-3 became the major type (46%), the previously predominant HAdV-4 decreased to 6 per cent, and HAdV-7 increased from 2 to 19 per cent .
The cause for this seasonal change in serotype is yet unknown. One hypothesis could be due to unknown environmental factors, such as humidity and temperature. Another could be possible favourable or unfavourable interactions with other circulating respiratory viruses. It may also be that a build-up of immunity in the population could contribute to the decline in a serotype; in contrast, waning population level immunity to certain serotypes may result in an increase in prevalence/incidence.
As we observed, the previous study at this laboratory showed that the majority, 70 (73%) of the 96 isolates, were from children ≤ 4 years of age . These findings are consistent with other studies that have also shown that pediatric populations with acute respiratory diseases are commonly infected with HAdV-1, HAdV-2, or HAdV-3 . Furthermore, the literature also demonstrates that most adult populations with HAdV infections are predominantly infected with HAdV-3, HAdV-4, HAdV-7 and HAdV-11, with a few HAdV-1, HAdV-2, HAdV-6 and HAdV-14 cases . Of note in this study, HAdV-11, HAdV-17 and HAdV-21 were only found among the adult population (age ≥ 18 years).
Adenovirus respiratory infections are highly prevalent in children; they have been documented in 5 to 11% of upper respiratory tract infections and bronchitis, 4 to 10% of pneumonia and pharyngitis, 2 to 10% of bronchiolitis, and 3 to 9% of croup in children . Most outbreaks of adenovirus respiratory infections, especially in children < 7 years of age, have been caused by HAdV-3, HAdV-4, HAdV-7 and HAdV-21 . Notably two of these serotypes, namely HAdV-3 and HAdV-21, have been circulating in the GTA for the past few years, as observed in this study and by Yeung et al.
From 1971 until 1996 the United States (US) had live oral adenovirus vaccines against HAdV-4 and HAdV-7 available for members of the military . The vaccines were effective in preventing adenovirus infections; in fact, the overall ARD levels decreased by 50% to 60% among military recruits . Studies conducted on the serum of vaccinated recruits have demonstrated a small cross-protective effect of the HAdV-4 and HAdV-7 vaccines against HAdV-3 and HAdV-14 [12, 13]. Following the withdrawal of the adenovirus vaccination program, there was an outbreak of HAdV-14 in US military training camps in 2006 . HAdV-14 had never been observed in a US recruit before 2006 [13, 14]. These outbreaks have sparked recognition of the importance of continued adenovirus surveillance as well as a need for a vaccine . In fact, due to a reoccurrence of HAdV outbreaks among military personnel, a second generation live oral vaccine against HAdV-4 and HAdV-7 was approved by the US Food and Drug Administration (FDA) in March 2011 . As of October 2011, this vaccine is being administered to US military recruits .
Our findings suggest that for the development of a new vaccine, more serotypes would have to be included, since the prevalence of HAdV serotypes may change. The rapid shift in serotypes from season to season suggests that a more successful vaccine will be serotype independent, and directed against common epitopes shared by all serotypes. Continued molecular epidemiological surveillance of circulating adenovirus serotypes is a critical tool in monitoring the changes in HAdV serotypes causing clinical disease.